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4.1 Lexical processing modules
4.1.1 Module description
4.1.2 Format processing Format encapsulation method

4 Modules specification

4.1 Lexical processing modules

4.1.1 Module description

One of the most efficient approaches to lexical processing is based on the use of finite-state transducers (FST) \cite[mohri97a]\cite[roche97b]. FST are a type of finite-state automata, which may be used as one-pass morphological analysers and generators and may be very efficiently implemented. In this project, we have used a class of FST called letter-transducers  [3]\cite[garrido02a] [4]; in fact, any finite-state transducer may always be turned into a letter-transducer. Garrido and collaborators  [4]\cite[garrido02a] give a formal definition of the letter transducers used in this project; describing them informally, a letter-transducer is an idealised machine consisting of:

  1. A (finite) set of states, that is, of situations in which the transducer can be while it is reading, from left to right, the input letters or symbols. Among the states of the set, we can distinguish:

    1. A single initial state: this is the state in which the transducer is before processing the first letter or the first symbol of the input.

    2. One or more acceptance states, which are only reached after having completely read a valid entry and, therefore, are used to detect valid words.

  2. A set (also finite) of state transitions consisting of:

    1. the origin state

    2. the destination state

    3. the input letter or symbol

    4. the output letter or symbol

    To make it possible that input and output have different lengths at any time, it is allowed that there is no input symbol, that there is no output symbol or that there is neither input nor output symbol. This case is generally represented using a special symbol (the empty symbol).

Every time the transducer reads an entry symbol, it creates a list of \emph{live} or \emph{active} states, each one of which has an associated output (a sequence of symbols). The way the letter transducer works is different for each type of lexical processing operation. For example, in the morphological analysis, the transducer tries to read the longest entry recognised by the dictionary (“left-to-right, longest-match” mode). \begin{enumerate} \item Beginning: the set of live states is given a single live state: the initial state, with the empty word ("") as output associated to the state. \item When from one of the states in the current set of live states it is possible to reach other states through transitions that do not have input symbol, these states are added to the set of live states, and are associated to the output obtained when extending the associated outputs with the output symbol found in the corresponding transitions. This expansion operation of the set of live states continues until it is not possible to add more states. \item A symbol from the input word is read. \item A new set of live states is created, made with the states reached through transitions that have that symbol as input, and this states are associated to the outputs extended by adding the corresponding output symbols found in the transitions. \item If the current set has any live state, the process continues on step 2. \item The sets of live states are read backwards until a set is found which contains acceptance states. The morphological analyses will be the outputs associated to these states, and the reading position is set to the position immediately after this set (so that it can be processed again by the transducer in the next pass). \end{enumerate} Not all acceptance states have the same characteristics, and this fact adds more conditions to the acceptance process, in order to be able to deal with unknown words or with words that are joined to other words, as will be explained later.

The transducer reads the input word only once on average, from right to left and symbol by symbol, and keeps a tentative list of possible partial outputs that is updated and pruned as the input is being read. When letter transducers are used as morphological analysers or as lemmatizers, they read a surface form and write the resulting lexical form(s). In this case, input symbols are the letters of the surface form, and output symbols are the letters needed to write the lemmas, as well as the letters and special symbols needed to represent the morphological analysis, such as in \texttt{<n>}, \texttt{<f>}, \texttt{<p2>}, etc.

The transducers work in a similar way for other lexical processing tasks.

\nota{La noció de LRLM (left-to-right, longest-match) (o ODSCML, "izquierda a derecha, recortando el segmento concordante más largo") ha de quedar clara en el funcionamient del morfològic i del trànsfer estructural. Afegir coses de l’article de EAMT 2005.}

\subsubsection{Letter case handling in dictionaries} \label{mayusc}

The same input word in a lexical processing module can be written differently regarding letter case. The most frequent cases are:

\begin{itemize} \item The whole word is in lower case. \item The whole word is in upper case. \item The first letter is capitalised and the rest is in lower case (typical case for proper nouns). \end{itemize}

The transductions in the dictionary can also be found in these three states. The way in which one word is written in the dictionary is used to discard possible analysis of the word, according to the following rules:

\begin{itemize} \item If the input letter is upper case and in the current analysis state there are concordant transitions in lower case, these transductions are made. \item If the input letter is lower case and in the current state there are not concordant transitions in lower case, the transductions are not made. \end{itemize}

Thanks to this policy, a surface form that is not capitalised can not be analysed as a proper noun.

The case of an input word will be maintained in the output of the translator unless it is decided not to do so. The case can be changed in the structural transfer module; this option is useful, for example, when there is a reordering of words or when a word is added before a capitalised word at the beginning of a sentence, such as in the translation of the Catalan phrase \emph{Vindran} into English: \emph{They will come}.

\subsection{Data format: the dictionaries} \label{ss:diccionarios} \subsubsection{General criteria for dictionary design}

The experience of the Transducens group at the Universitat d’Alacant in the creation of machine translation systems between Romance languages (\texttt{es}, \texttt{ca} and \texttt{pt}) already operative and publicly accessible has inspired the main characteristics of the whole shallow-transfer machine translation system described in this document, as well as its application to the Romance languages of Spain (\texttt{es}, \texttt{ca} and \texttt{gl}). In some sense, it could be stated that in the present project the only work was to adapt (rewrite in a standardised and interoperable format) the specifications and programs used in already operative projects.

In particular, the design of the dictionaries has been based in an architecture that pretends to separate, as far as possible, the source language from the target language, even knowing that these dictionaries are translation-oriented and, therefore, that it is not advisable to elaborate them completely separately. The chosen format is used for the specification of both morphological dictionaries (monolingual) and bilingual dictionaries.

The format for dictionaries, as well as for the rest of linguistic data (definition file for part-of-speech tagger and structural transfer rules) is XML\footnote{\url{http://www.w3.org/XML/}}, an international standard used in numerous natural language processing projects which, thanks to the availability of many utilities and libraries, it is becoming a very powerful tool for linguistic data representation and exchange (see article \cite{ide00}).

Dictionaries are designed so that they can be compiled into \textit{letter transducers }, for efficiency reasons. For more information on letter transducers as a particular case of finite-state transducers, see Section \ref{ss:funcproclex} or the article \cite{garrido02a}.

The letter transducers that are generated from the system dictionaries (morphological, bilingual and post-generation dictionaries) process input character strings to produce output strings. According to this, dictionaries are made of entries consisting of string pairs that correspond to the inputs and outputs of the transducer.

The most powerful tool in these dictionaries is the definition and use of \emph{paradigms}. Since in Romance languages a lot of lemmas share the same inflection pattern (there are regularities in their inflection), it is useful and straightforward to group these regularities in inflection paradigms to avoid having to write all the forms of every word. Paradigms allow the representation of dictionary entries compactly and help optimise the speed for building a dictionary. Once the most frequent paradigms in a dictionary are defined, the linguist does not need to bother, in most cases, with the whole inflection of a new term, since entering an inflective word is generally limited to writing the lemma and choosing one inflection pattern among the previously defined paradigms. Furthermore, the use of paradigms reduces the memory requisites, facilitates the construction of efficient letter transducers and speeds up the compilation process \cite{ortiz05j}. We did not use paradigms in bilingual dictionaries (although it is possible to) because most of the inflection information is processed implicitly in these dictionaries, as explained in page~\pageref{ss:bil}.

\subsubsection{Dictionary types}

In our system there are three types of dictionaries: morphological (monolingual) dictionaries for each of the languages involved (Spanish, Catalan and Galician); bilingual dictionaries for the different translation pairs (Spanish–Catalan and Spanish–Galician), and post-generation dictionaries for each of the languages (a post-generation dictionary is not a typical dictionary, with lemmas and morphological information, but is like a little dictionary of the orthographic transformations that words may undergo when they come together). The structure of the three dictionary types is specified by the same DTD (\emph{Document Type Definition}), which can be found in Appendix \ref{ss:dtd_dics}.

\textbf{Morphological dictionaries} are used both for building morphological analysers —the translation system module used to obtain all the possible lexical forms for a certain surface form in the source language — and morphological generators —the module that generates the surface form in the target language from the lexical form of each word—. These two modules are obtained from a single morphological dictionary, depending on the direction in which it is read by the system: read from left to right, we obtain the analyser, and read from right to left, the generator.

The block structure typical for these dictionaries is the following:

\begin{itemize} \item \textit{An alphabet definition}. This definition is used exclusively for building the morphological analyser; specifically, it enables the morphological analyser to appropriately tokenize unknown words and the ones in the conditional sections (see the description of the element \texttt{<section>} in page \pageref{ss:section}); the morphological generator does not need this definition.

\item \textit{A definition of symbols}. It consists of a declaration of the grammatical symbols that will be used in dictionary entries (you can find in Appendix \ref{se:simbolosmorf} a list with the grammatical symbols used in this project). \item \textit{A definition of paradigms}. Paradigms need to be defined here in order to be used in the dictionary sections or in other paradigms. \item \textit{One or more dictionary sections with conditional tokenization}, type \texttt{standard}. To include most of the words of the dictionary. \item \textit{One or more dictionary sections with unconditional tokenization}. To include certain words that follow a regular pattern or that are tokenized regardless the text directly after them (see description of the element \texttt{<section>} in page \pageref{ss:section}). In the Catalan morphological dictionaries, words requiring an unconditional tokenization are distributed in two sections: one for the forms that require the introduction of a blank immediately after (due to processing requirements of the lexical forms), like the apostrophized forms \emph{l’} or \emph{d’}, and another one for punctuation marks, numbers and other signs.


\textbf{Bilingual dictionaries} represent in the system the lexical transfer process, that is, the assignment of the TL lexical form that corresponds to each SL lexical form. Two \emph{products} are obtained from each bilingual dictionary, depending on the direction in which it is read by the system: when the dictionary is read from left to right, we obtain the lexical transfer module in one translation direction, and when it is read from right to left, in the other direction. For the bilingual dictionaries of our project, it has been established that Spanish will be put always on the left side of the entries, and the rest of the languages (Catalan and Galician), on the right side. Thus, for example, the bilingual Spanish–Galician dictionary will be read from left to right for the translation \texttt{es}–\texttt{gl} and from right to left for the translation \texttt{gl}–\texttt{es}. In applications like the ones in this project, these dictionaries do not have paradigms: they are build with generic entries which almost always have no more information than lemma and part of speech, and there is no inflection information.

The block structure used in the bilingual dictionaries of this project is the following:

\begin{itemize} \item \textit{A definition of symbols}. It consists of a declaration of the grammatical symbols that will be used in dictionary entries. \item \textit{A single dictionary section}. Where bilingual correspondences are specified. \end{itemize}

Since 2007, bilingual dictionaries allow the specification of more than one TL translation, so that a lexical selection module (see Section \ref{se:seleccio_lex}) can choose the most suitable equivalent according to the context. To that end, an attribute has been added to bilingual dictionaries. You can find its description in section \ref{dic_lextor}.

\textbf{Post-generation dictionaries} are used to perform some transformations (orthographic changes, contractions, apostrophation, etc.) required after surface forms in the target language have been generated and come into contact with each other. Since this kind of operation can be expressed as a translation of character strings, it has been decided to use the same type of dictionaries. It is implicitly assumed that the parts of the text whose processing has not been specified are copied just as they arrive. In these dictionaries, the definition of paradigms is useful to express systematic changes in the word contact phenomena. Unlike the other dictionary types, these do not include grammatical symbols, since they process surface forms.

The block structure of post-generation dictionaries is the following: \begin{itemize}

\item \textit{A definition of paradigms}. To use in entries. \item \textit{A dictionary section}. Where the patterns for post-generation operations are specified. \end{itemize}

The following table contains an overview of the possible reading directions of dictionaries and their application to the Romance languages in this project:

\begin{center} \begin{tabular}{|l|l|l|} \hline Dictionary & Reading direction & Function \\ \hline Morphological & left–right & analysis for \texttt{es}, \texttt{ca} and \texttt{gl}\\ & right–left & generation for \texttt{es}, \texttt{ca} and \texttt{gl}\\\hline Bilingual & left–right & translation for \texttt{es-ca} and \texttt{es-gl}\\ & right–left & translation for \texttt{ca-es} and \texttt{gl-es}\\\hline Post-generation & left–right & post-generation for \texttt{ca}, \texttt{es} and \texttt{gl}\\\hline

\end{tabular} \end{center}

\subsubsection{Description of the dictionary format} \label{formatodics} This section presents the main elements of the format in which dictionaries are built. The formal definition (a DTD) can be found in Appendix ~\ref{ss:dtd_dics}. Section \ref{dic_lextor} describes the characteristics of a bilingual dictionary that works in an Apertium system with lexical selection module. Finally, from pages \pageref{ss:morfgen} to %\pageref{ss:bil} y \pageref{ss:postgen} there is a description of the different particularities of entries for the three dictionary types (morphological, bilingual and post-generation).

\paragraph{Element for dictionary \texttt{<dictionary>}}

This is the root element and includes the whole dictionary. It contains an alphabetic character definition, a definition of symbols (which are the morphological tags for the words), a definition of inflection paradigms and one or more dictionary sections, which contain the entries for the lexical forms (consisting of pairs made of surface form–lexical form). Figure \ref{fig:dictionary} shows the basic block structure of a generic dictionary.

\begin{figure} \begin{small} \begin{alltt} <?\textbf{xml} \textsl{version}="1.0" \textsl{encoding}="utf-8"?> <\textbf{dictionary}> <\textbf{alphabet}>abcdefghijk ... ABCDEFGH ... çñáéíóú</\textbf{alphabet}> <\textbf{sdefs}> <!– ... –> </\textbf{sdefs}> <\textbf{pardefs}> <!– ... –> </\textbf{pardefs}> <\textbf{section} ...> <!– ... –> </\textbf{section}> <!– ... –> </\textbf{dictionary}> \end{alltt} \end{small} \caption{Use of the elements \texttt{<\textbf{dictionary}>} and \texttt{<\textbf{alphabet}>}} \label{fig:dictionary} \end{figure}

\paragraph{Element for alphabet \texttt{<alphabet>}}

It is used to specify a definition of alphabetic characters. The purpose of this specification is enabling the modules that process the input by means of letter transducers to tokenize it in individual words.\nota{Parlar dels mots desconeguts. Cita \ref{ss:section} - Mikel?}

In the present design, the definition of an alphabet only has sense in morphological dictionaries, since it is needed for the analysis. Figure \ref{fig:dictionary} shows a use example for this element.

\paragraph{Element for symbol definition section \texttt{<sdefs>}} It groups all the symbol definitions in a dictionary (\texttt{<\textbf{sdef}>}). There is an example of its use in Figure \ref{fig:sdefs}.

\paragraph{Element for symbol definition \texttt{<sdef>}}

It is an empty element (it does not delimit any content): it is used to specify, through the values of the attribute \texttt{\textsl{n}}, the names of the grammatical symbols that are used in the dictionary to morphologically label lexical forms. In Figure \ref{fig:sdefs} you can find a use example for this element. Refer to Appendix \ref{se:simbolosmorf} if you need a list with all the grammatical symbols used in the dictionaries of this project.

\begin{figure} \begin{small} \begin{alltt} <\textbf{sdefs}> <\textbf{sdef} \textsl{n}="n"/> <\textbf{sdef} \textsl{n}="det"/> <\textbf{sdef} \textsl{n}="sg"/> <\textbf{sdef} \textsl{n}="pl"/> <!– ... –> </\textbf{sdefs}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{sdefs}>}} \label{fig:sdefs} \end{figure}

\paragraph{Element for dictionary section \texttt{<section>}} \label{ss:section}

It contains the words that will be recognised by the dictionary. The reason to divide a dictionary in sections is that some forms —for example, the ones coming from the identification of certain regular patterns, or some forms that pertain to a specific dialect— may need a different processing.

One of the problems that the definition of sections in a dictionary helps to solve is the tokenization procedure during morphological analysis. Most of the forms are tokenized following a conditional criterion: identifying if the character being processed is followed by a non-alphabetic character —that is, not defined in \texttt{<\textbf{alphabet}>}—. However, there are other forms, like the Catalan apostrophized words \emph{l’} or \emph{d’}, that need an unconditional tokenization model: there is no need to analyse what comes after them, since, if it is an alphabetic character, it will belong to the \textit{next} word. The forms that require unconditional tokenization are included in a specific section of the dictionary. Other kinds of processing can also be solved through these divisions.

The value of the attribute \texttt{\textsl{type}} is used to express the kind of string tokenization applied in each dictionary section: the possible values of this attribute are: \texttt{standard}, for almost all the forms of the dictionary (conditional mode), \texttt{preblank} and \texttt{postblank}, for the forms that require an unconditional tokenization and the placing of a blank (before and after, respectively), and \texttt{inconditional} for the rest of forms that require unconditional tokenization.

The attribute \texttt{\textsl{id}} is used to assign an identifier (a name) to the dictionary sections.

\begin{figure} \begin{small} \begin{alltt} <\textbf{section} \textsl{id}="principal" \textsl{type}="standard"> <!– ... –> </\textbf{section}> <\textbf{section} \textsl{id}="patterns" \textsl{type}="inconditional"> <!– ... –> </\textbf{section}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{section}>}} \label{fig:section} \end{figure}

\paragraph{Element for entries \texttt{<e>}}

An entry is the basic unit of a dictionary or of a paradigm definition. Entries consist of a concatenation in any order of string pairs \texttt{<\textbf{p}>}, identity transductions \texttt{<\textbf{i}>}, references to paradigm \texttt{<\textbf{par}>} or regular expressions \texttt{<\textbf{re}>}. The structure and meaning of these elements is explained later in this section (in pages ~\pageref{ss:p}, \pageref{ss:i}, \pageref{ss:par} and \pageref{ss:re} respectively).

\label{restric}Two optional attributes are used with this entry. The first one is \texttt{\textsl{r}} (for \textit{restriction}), which specifies if the entry has to be considered only when reading the dictionary from left to right (\texttt{LR}) or when reading it from right to left (\texttt{RL}). If nothing is specified, it is assumed that the entry must be considered in both directions.

In morphological dictionaries, the restriction \texttt{LR} causes that a LF is analysed but not generated (for example, when the LF belongs to a dialectal variant that we wish to recognise but not to generate) and the restriction \texttt{RL} causes that a word is generated but not analysed (needed, for example, for forms with post-generator activation mark, see page \pageref{ss:a} for more details).

In bilingual dictionaries, the restrictions \texttt{LR} and \texttt{RL} cause that the translation is done only in one direction: for example, in a bilingual \texttt{es}–\texttt{ca} dictionary, \texttt{LR} indicates that the LF is only translated from Spanish to Catalan, and \texttt{RL} only from Catalan to Spanish. Let’s illustrate it with an example: the Spanish adverbs \emph{aún} and \emph{todavía} ("still") are translated into Catalan as the same word, \emph{encara}. We can only translate the Catalan adverb \emph{encara} as one of both words into Spanish (there is no difference in meaning); we decide to translate it as \emph{todavía}. In this case, we have to write two entries in the bilingual dictionary: the entry that matches \emph{aún} with \emph{encara} needs to have the restriction \texttt{LR} (translation only from \texttt{es} to \texttt{ca}) and the one that matches \emph{todavía} with \emph{encara} does not need to have any restriction (translation in both directions).

Direction restrictions are also necessary in bilingual dictionaries when we have words with gender to be determined ("GD") or number to be determined ("ND") (consult page ~\pageref{ss:bil} for more information).

The other optional attribute in entries is the lemma name \texttt{\textsl{lm}}. Due to the employment of paradigms to represent the inflection regularities of lexical units, an entry in morphological dictionaries contains the part of the lemma that is common to all the inflected forms, that is, it contains the lemma cut at the point in which the paradigm regularity begins (for example, the Spanish adjectives \emph{distinto}, \emph{absoluto} and \emph{marino} appear in entries as \emph{distint}, \emph{absolut} and \emph{marin}, since the rest of the inflected forms is common to all of them and specified in a paradigm). This fact can make the dictionary difficult to understand. Therefore entries have this attribute, which contains the whole lemma of the lexical form, so that the dictionary becomes more understandable and linguists can solve problems quickly. In bilingual dictionaries, which normally do not have references to paradigms,\footnote{They could have references to paradigms, but we did not judge it necessary for the languages involved \nota{atenció: ex–, vice–?}.} this attribute is not used.

\paragraph{Element for string pair \texttt{<p>}} \label{ss:p}

This basic element of dictionaries is used in any kind of entry to indicate the correspondence between two strings; this correspondence specifies a lexical transformation that will be carried out by a state path in the resulting finite-state transducer \cite{garrido99j}.

It is defined by a pair of internal elements: The left element (\texttt{<\textbf{l}>}) and the right element (\texttt{<\textbf{r}>}). Its structure is shown in Figure \ref{fig:p}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{p}> <\textbf{l}><!– ... –></\textbf{l}> <\textbf{r}><!– ... –></\textbf{r}> </\textbf{p}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{p}>}} \label{fig:p} \end{figure}

A pair \texttt{<\textbf{p}>} must include these two parts although one can be empty, which means deleting (or inserting) a string. The elements \texttt{<\textbf{l}>} and \texttt{<\textbf{r}>} have the same internal structure and the same requisites. They can contain text and references to grammatical symbols (which, for the languages of the present project, inflected by suffixation, are usually placed at the end in any amount). Outside the tags \texttt{<\textbf{l}>} and \texttt{<\textbf{r}>} of a string pair there is nothing.

\paragraph{Element for reference to symbol \texttt{<s>}}

References to symbols (or tags) are used to specify the morphological information of a LF and are used in any place inside a string pair, that is, inside the elements \texttt{<\textbf{l}>} and \texttt{<\textbf{r}>}, as if they were individual characters; for the languages of our project, however, they are put at the end of the pairs and always in the same order for the same word type. This order is decided by the linguist according to how he/she wishes to characterise morphologically the LF in the dictionaries, and must be the same in all the dictionaries of a system if we want that the lexical and structural transfer operations work correctly. So, for example, in the Romance language dictionaries of this project, a noun has in the first place the symbol for part of speech (\textit{n}, noun), then for gender (\textit{m}, masculine, \textit{f}, feminine, \textit{mf}, masculine–feminine), and finally for number (\textit{sg}, singular, \textit{pl}, plural, \textit{sp}, singular–plural). The list in Appendix \ref{se:simbolosmorf} contains all the grammatical symbols used in the dictionaries of this project and shows the order which has been established for each type of word.

In morphological dictionaries, references to symbols are used in paradigms as well as in entries which do not include any reference to a paradigm. In bilingual dictionaries, usually only the first symbol of each LF is specified, since the rest is automatically copied from the source language LF to the target language LF (in the case they are identical in both languages).

To specify which symbol we are referring to, we use the (mandatory) attribute \texttt{\textsl{n}}. The symbol must be defined in the symbol definition section (\texttt{<\textbf{sdefs}>}).

\paragraph{Element for identity transduction \texttt{<i>}} \label{ss:i}

It is a way to write a string pair in which left side and right side are identical. For example, the two entries shown in Figure \ref{fig:i} are completely equivalent. The advantage of writing entries with this element is that the result is more compact and more readable.

\begin{figure} \begin{small} \begin{alltt} [1]

<\textbf{e} \textsl{lm}="perro"> <\textbf{p}> <\textbf{l}>perr</\textbf{l}><\textbf{r}>perr</\textbf{r}> </\textbf{p}> <\textbf{par} \textsl{n}="abuel/o__n"/> </\textbf{e}>


<\textbf{e} \textsl{lm}="perro"> <\textbf{i}>perr</\textbf{i}> <\textbf{par} \textsl{n}="abuel/o__n"/> </\textbf{e}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{i}>} entries [1] and [2] are equivalent} \label{fig:i} \end{figure}

\paragraph{Element for paradigm definition section \texttt{<pardefs>}}

This element includes all the paradigm definitions of a dictionary, each definition in an element \texttt{<\textbf{pardef}>}, as shown in Figure \ref{fig:pardefs}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{pardefs}> <\textbf{pardef} \textsl{n}="abuel/o__n"> <!– ... –> </\textbf{pardef}> <!– ... –> </\textbf{pardefs}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{pardefs}>}} \label{fig:pardefs} \end{figure}

\paragraph{Element for paradigm definition \texttt{<pardef>}}

It defines an inflection paradigm in the dictionary. A paradigm can be understood as a small dictionary of alternative transformations that can be concatenated to parts of words (or to entries of another paradigm) to specify regularities in the lexical processing of the dictionary entries, such as inflection regularities. To specify these regularities, each paradigm is a list of entries \texttt{<\textbf{e}>} like the ones in the dictionary, that is, it has the same structure as a dictionary section \texttt{<\textbf{section}>}; therefore, paradigm entries consist of a pair (\texttt{<\textbf{p}>}) with left side (\texttt{<\textbf{l}>}) and right side (\texttt{<\textbf{r}>}). These elements can contain text or grammatical symbols \texttt{<\textbf{s}>}.

As in symbol definitions, paradigm definitions have an attribute \texttt{\textsl{n}} which specifies the paradigm name, so that it can be referred to inside dictionary entries. In a dictionary entry, therefore, one only needs to indicate the corresponding paradigm name in order that all its possible forms get specified.

The example of paradigm definition pointed out in Figure \ref{fig:pardefs} appears developed in Figure \ref{fig:pardef}. The following table shows the information expressed by the paradigm:

\begin{center} \begin{tabular}{|l|c|l|} \hline Root (SF and LF) & Ending (SF) & Analysis (LF) \\ \hline \texttt{abuel} & \texttt{o} &\texttt{o<n><m><sg>}\\ \texttt{abuel} & \texttt{a} &\texttt{o<n><f><sg>}\\ \texttt{abuel} & \texttt{os} &\texttt{o<n><m><pl>}\\ \texttt{abuel} & \texttt{as} &\texttt{o<n><f><pl>}\\ \hline \end{tabular} \end{center}

\begin{figure} \begin{small} \begin{alltt} <\textbf{pardef} \textsl{n}="abuel/o__n"> <\textbf{e}> <\textbf{p}> <\textbf{l}>o</\textbf{l}> <\textbf{r}>o<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="sg"/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e}> <\textbf{p}> <\textbf{l}>a</\textbf{l}> <\textbf{r}>o<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="f"/><\textbf{s} \textsl{n}="sg"/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e}> <\textbf{p}> <\textbf{l}>os</\textbf{l}> <\textbf{r}>o<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="pl"/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e}> <\textbf{p}> <\textbf{l}>as</\textbf{l}> <\textbf{r}>o<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="f"/><\textbf{s} \textsl{n}="pl"/></\textbf{r}> </\textbf{p}> </\textbf{e}> </\textbf{pardef}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{pardef}>} to define the inflective morphology of Spanish nouns with four endings, such as \emph{abuelo, -a, -os, -as} ("grandfather, grandmother") } \label{fig:pardef} \end{figure}

This paradigm is assigned to all Spanish nouns (\texttt{n}) that inflect like \emph{abuelo}, such as \emph{alumno}, \emph{amigo} or \emph{gato}, and is designed to be used as a \textit{suffix} in dictionary entries. In general, paradigms can be applied to any position of a dictionary entry (if it makes sense, of course). We can think of paradigms as transducers that are inserted at the point where they are specified. Figure \ref{fig:pardef2} shows an example of paradigm defined to be used as a prefix. It is the paradigm used to analyse and generate Spanish words beginning with \emph{ex}, \emph{ex-}, etc., like \emph{ex-presidente}, \emph{exministro}, \emph{ex director}, etc., with all the orthographic variations (\emph{ex} with hyphen, without hyphen and joined, without hyphen and with a blank \texttt{<\textbf{b}/>}, see page~\ref{s3:b}); the output lemma simply adds \emph{ex} without hyphen nor blank to the accompanying lemma. The direction restrictions (\texttt{"LR"}) that appear in the example are used to determine which form will the translator generate. The empty identity transduction (\texttt{<\textbf{i}/>}) is necessary in this case to analyse and generate the word without the prefix \emph{ex}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{pardef} \textsl{n}="ex"> <\textbf{e} \textsl{r}="LR"><\textbf{p}><\textbf{l}>ex<\textbf{b}/></\textbf{l}><\textbf{r}>ex</\textbf{r}></\textbf{p}></\textbf{e}> <\textbf{e}><\textbf{i}>ex</\textbf{i}></\textbf{e}> <\textbf{e} \textsl{r}="LR"><\textbf{p}><\textbf{l}>ex-</\textbf{l}><\textbf{r}>ex</\textbf{r}></\textbf{p}></\textbf{e}> <\textbf{e}><\textbf{i}/></\textbf{e}> </\textbf{pardef}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{pardef}>} in the paradigm for the prefix \emph{ex}.} \label{fig:pardef2} \end{figure}

Entries in a paradigm can contain references to other paradigms provided that these have been defined upper in the file. On the other hand, for the moment a paradigm definition can not include itself neither directly nor indirectly.

Paradigms are used in morphological dictionaries for the analysis and generation of lexical forms. For the language pairs of this project, there is no need to define paradigms in bilingual dictionaries (see page~\pageref{ss:bil}).

From Apertium 2 on, there is a new type of paradigm, called metaparadigm, that allows the definition of paradigms with variations according to the value of an attribute specified in each entry that refers to that paradigm. Section \ref{ss:metaparadigmas} describes the characteristics and use of metaparadigms.

\paragraph{Element for reference to a paradigm \texttt{<par>}} \label{ss:par}

It is used inside an entry to indicate which inflection paradigm, among the ones defined in \texttt{<\textbf{pardefs}>}, follows the entry. Thanks to the references to paradigms there is no need to write all the inflected forms of a lemma in a morphological dictionary entry. The attribute \texttt{\textsl{n}} is used to specify the name of the paradigm we want to refer to.

The result of inserting a reference to a paradigm in an entry is the creation of so many string pairs as cases specified in the paradigm. For example, the entry in Figure \ref{fig:par}, with a reference to the paradigm "\texttt{abuel/o\_\_n}" (defined in Figure \ref{fig:pardef}), is equivalent to an entry where each string pair of the paradigm is concatenated to the lemma (that is, an entry with every inflected form of the lemma), as shown in Figure \ref{fig:lema_par}. In this figure, you can see that the paradigm delivers always in the right string (\texttt{<\textbf{r}>}) the lemma (\emph{perro}) with the grammatical symbols that apply to the surface form, since it is from the lemma that transfer operations are carried out.

The appropriate use of paradigms, besides enabling the creation of compact dictionaries, improves compilation speed and reduces memory requirements during this process, since in compilation it is possible to create a single data structure for each one of most paradigms \cite{ortiz05j}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="perro"> <\textbf{i}>perr</\textbf{i}> <\textbf{par} \textsl{n}="abuel/o__n"/> </\textbf{e}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{par}>}} \label{fig:par} \end{figure}

\begin{figure} \begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>perro</\textbf{l}> <\textbf{r}>perro<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="sg"/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e}> <\textbf{p}> <\textbf{l}>perra</\textbf{l}> <\textbf{r}>perro<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="f"/><\textbf{s} \textsl{n}="sg"/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e}> <\textbf{p}> <\textbf{l}>perros</\textbf{l}> <\textbf{r}>perro<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="pl"/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e}> <\textbf{p}> <\textbf{l}>perras</\textbf{l}> <\textbf{r}>perro<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="f"/><\textbf{s} \textsl{n}="pl"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{Entry equivalent to the one in Figure \ref{fig:par}, that shows the result of inserting the reference to paradigm \texttt{<\textbf{par}>} with the paradigm defined in Figure \ref{fig:pardef}.} \label{fig:lema_par} \end{figure}

\paragraph{Element for regular expression \texttt{<re>}} \label{ss:re}

In natural languages too there are patterns that can be recognized as regular expressions: for example, punctuation marks, numbers (Latin or Roman), e-mail or web page addresses, or any kind of code identifiable through these mechanisms.

For this cases we use the string contained in the tag \texttt{<\textbf{re}>}. The compiler reads the regular expression definition and transforms it in a transducer that is inserted in the rest of the dictionary and that translates all the strings that match the expression into identical strings.

The syntax of the present implementation of these regular expressions processes a subgroup of Unix regular expressions, which includes the operators \texttt{*}, \texttt{?}, \texttt{|} and \texttt{+}, as well as groupings through parentheses and optional character ranks, for example \texttt{[a-zA-zñú]} or its negated versions, like \verb![^a-z]!.

By analogy, they can be seen as \texttt{<\textbf{i}>} elements, with the difference that they can identify strings which may be infinite (like numbers).

\begin{figure} \begin{small} \begin{alltt} <\textbf{e}> <\textbf{re}>[0-9]+([.,][0-9]+)?(\%)?</\textbf{re}> <\textbf{p}><\textbf{l}/><\textbf{r}><\textbf{s} \textsl{n}="num"/></\textbf{r}></\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{Us of the element \texttt{<\textbf{re}>} in an entry for the detection of Arabic numbers.} \label{fig:e} \end{figure}

Figure \ref{fig:e} shows the way to tag quantities expressed as Arabic numbers in the dictionary.

\paragraph{Element for blank block \texttt{<b>}} \label{s3:b}

It is used to express the presence of blanks between the words of a multiword (see page~\pageref{ss:multipalabras} for an explanation on multiwords). It can be inserted in the \texttt{<\textbf{i}>}, \texttt{<\textbf{l}>} and \texttt{<\textbf{r}>} elements. In Figure \ref{fig:b} you can see the entry for the Spanish multiword expression \emph{hoy en día} ("nowadays"): the blanks between words are expressed as \texttt{<\textbf{b}/>} elements inside the left and right strings. \begin{figure} \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="hoy en día"> <\textbf{p}> <\textbf{l}>hoy<\textbf{b}/>en<\textbf{b}/>día</\textbf{l}> <\textbf{r}>hoy<\textbf{b}/>en<\textbf{b}/>día<\textbf{s} \textsl{n}="adv"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{b}>}} \label{fig:b} \end{figure}

Blanks can consist of normal space characters or of document format information blocks encapsulated by the de-formatter (\textit{superblanks}, see Section \ref{ss:formato}).

\paragraph{Element for post-generator activation \texttt{<a>}} \label{ss:a} The element \texttt{<\textbf{a}>} for the activation of the post-generator is used to indicate that a word in target language may undergo orthographic transformations due to the contact with other words; for example, being apostrophized, contracted, written without intermediate spaces, etc. These transformations need be carried out after the generation of the target language surface forms, as until then words are isolated and it is not possible to know which words will get in contact . Therefore, these operations must be carried out by the module next to the generator, which is called post-generator. In order to signal which words are to be processed by the post-generator, this element is used in the surface form side of these entries in the morphological dictionary.

The example in Figure \ref{fig:a} shows its use, in a Catalan morphological dictionary, for the preposition \textit{de}, which, when appearing before a singular or plural masculine definite article (\textit{el, els}), forms a contraction (\textit{del, dels}). The presence of the tag \texttt{<\textbf{a}/>} causes the activation of the post-generator, which checks whether the preposition is followed by one of the words that cause it to contract and, if it is so, makes the contraction (see page~\pageref{ss:postgen} for more details). The restriction \texttt{RL} indicates that this is an only-generation entry, since it does not make any sense for the analysis.

\begin{figure} \begin{small} \begin{alltt} <\textbf{e} \textsl{r}="RL" \textsl{lm}="de"> <\textbf{p}> <\textbf{l}><\textbf{a}/>de</\textbf{l}> <\textbf{r}>de<\textbf{s} \textsl{n}="pr"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{a}>} in a morphological dictionary} \label{fig:a} \end{figure}

\paragraph{Element for group marking \texttt{<g>}}

This element is used, inside the \texttt{<\textbf{l}>} and \texttt{<\textbf{r}>} elements, to define groups that require a special treatment beyond the normal word by word processing. It is used in inflective multiwords to signal the beginning and the end of the group of invariable lexical forms (one or more) that are adjacent to the inflected word and that, together with it, build an inseparable unit. In Section~\ref{ss:multipalabras} you will find a detailed explanation of the different multiword types, and in Figure \ref{fig:hacertilin} of that section you can see an example of its use.

\paragraph{Element for joining of lexical forms \texttt{<j>}} \label{ss:j}

This element is used only in the right side of an entry (\texttt{<\textbf{r}>}) to indicate that the words that form a multiword are treated as individual lexical forms and, therefore, have a grammatical symbol each. This way, this multiword will be processed as a unit by the analyser and by the tagger until it reaches the auxiliary module \texttt{pretransfer} (see section \ref{se:pretransfer}), which is responsible for separating the lexical forms it is made of so that they reach the transfer module as independent forms. If the linguist wants that these forms reach the generator as joined forms, building again a multiword, it is necessary to define a structural transfer rule that groups them in a multiword (see Section \ref{formatotransfer}). If, on the contrary, these joined forms must be only for the analysis, the entry must have the restriction \texttt{LR}.

In Section~\ref{ss:multipalabras} you will find a more detailed explanation of this element. An example of its use can be found in Figure \ref{fig:cont} of the mentioned section.

\subsubsection{Modification of bilingual dictionaries for the new lexical selection module} \label{dic_lextor}

In 2007, a new module has been added to the Apertium system: the lexical selection module, which is described in section \ref{se:seleccio_lex}.

In order for them to work in a lexical selection system, bilingual dictionaries must be slightly modified so that they allow the specification of more than one translation in target language. The only change is the addition of two new attributes to the element \texttt{<e>}. Although these new attributes can be used in all the dictionaries of a system, they only make sense in a bilingual dictionary entry.

In Appendix~\ref{dixdtd} there is the part of the DTD \texttt{dix.dtd} \nota{MG: no caldria ajuntar les dues DTDs en una de sola?} where the element \texttt{e} used for dictionary entries is defined. The new attributes are: \begin{description} \item[slr (\emph{sense from left to right})] is used to specify the \emph{translation mark} when there is more than one translation from left to right for the lemma specified in the left side of an entry. The attribute can receive any value; however, the recommended action is to assign as value the lemma contained in the right part \texttt{<r>} (the translation of the lemma). \item[srl (\emph{sense from right to left})] is used to specify the \emph{translation mark} when there is more than one translation from right to left for the lemma specified in the right side of an entry. As before, the attribute can receive any value, but the recommended action is to assign as value the lemma contained in the left part \texttt{<l>} (the translation of the lemma). \end{description}

Furthermore, in both cases the value of the attribute can end in a white space and the letter “D” to indicate that this is the default translation, that is, the translation that will be chosen when there is not enough information to make a decision. It is compulsory that, for entries that have more than one equivalent in target language, one of the equivalents, and only one, is marked with the letter “D” for \emph{default}.

The following example shows how the new attributes are used. We take as example a bilingual English-Catalan dictionary, with the following entries having more than one translation in the target language: \begin{itemize} \item \emph{look}: can be translated into Catalan as \emph{mirar} (default) or as \emph{semblar} (according to the English senses \emph{view/seem}), \item \emph{floor}: can be translated into Catalan as \emph{pis} (default) or as \emph{terra} (according to the English senses \emph{level of building/ground}), \item \emph{pis}: can be translated into English as \emph{flat} (default) or as \emph{floor}. \end{itemize}

This information is represented by means of the two attributes described:\label{entrades_lextor} \begin{alltt} \begin{small} <e srl="flat D"> <p> <l>flat<s n="n"/></l> <r>pis<s n="n"/><s n="m"/></r> </p> </e>

<e slr="pis D" srl="floor"> <p> <l>floor<s n="n"/></l> <r>pis<s n="n"/><s n="m"/></r> </p> </e>

<e slr="terra"> <p> <l>floor<s n="n"/></l> <r>terra<s n="n"/><s n="m"/></r> </p> </e>

<e slr="mirar D"> <p> <l>look<s n="vblex"/></l> <r>mirar<s n="vblex"/></r> </p> </e>

<e slr="semblar"> <p> <l>look<s n="vblex"/></l> <r>semblar<s n="vblex"/></r> </p> </e> \end{small} \end{alltt}


\subsubsection{Particularities of the different dictionary types} \label{ss:morfgen}

Dictionary entries have different characteristics depending on the dictionary type. Although some of these characteristics have been presented in the previous sections, we are going to describe them here more exhaustively.

\paragraph{Morphological dictionaries}

In these dictionaries, used to generate the system’s morphological analysers and generators, it is necessary to mark with \texttt{<\textbf{a}/>} those surface forms which, once generated, may need certain orthographic transformations due to the contact with other words; these operations are carried out by the post-generator. As these marks are only generated, the entries containing them must be only for the generation, which means that need to have the restriction \texttt{\textsl{r}=}\verb!"RL"! (from right to left). Figure \ref{fig:a} shows an entry containing this element.

\paragraph{Bilingual dictionaries} \label{ss:bil}

As explained before, we have not used paradigms in the bilingual dictionaries of our system; these dictionaries are built with generic entries in which, almost always, only part of speech is specified, and which do not have inflection information. For example, in the \texttt{es-ca} dictionary, the entry for the Spanish words \textit{pan}, \textit{panes} ("bread"), translated into Catalan as \textit{pa}, \textit{pans}, would be as shown in Figure \ref{fg:pan}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>pan<\textbf{s} \textsl{n}="n"/></\textbf{l}> <\textbf{r}>pa<\textbf{s} \textsl{n}="n"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{Bilingual dictionary entry for the translation \emph{pan} (\texttt{es})–\emph{pa} (\texttt{ca})} \label{fg:pan} \end{figure}

As you can see in the figure, only the first grammatical symbol \texttt{<\textbf{s} \textsl{n}="\ldots}\texttt{"}\texttt{/>} of each word is specified, since the unspecified symbols that come after the specified ones in the bilingual dictionary are copied from the source lexical form to the target lexical form. This entry, therefore, works both for \textit{pan} (singular) and for \textit{panes} (plural): the morphological analyser delivers the lemma (\emph{pan}) followed by the grammatical symbols that apply to the analysed surface form (\emph{n m sg} or \emph{n m pl} as applicable), and the symbols that are not specified in the bilingual entry (\emph{m sg} or \emph{m pl}) are copied to the target language. This is valid for both translation directions. The idea is to specify the information indispensable to differentiate the entries, and the rest is \textit{deduced} (copied). It is important to bear this in mind, because, when there are differences between the grammatical symbols of a lexical form from SL to TL, these differences must be specified in the bilingual dictionary. For example, when between source word and translated word there is a gender or number change, one has to specify the grammatical symbols in order (the order in which these symbols appear in the morphological dictionaries)\footnote{To know which grammatical symbols have been used in the dictionaries and in which order, see Appendix \ref{se:simbolosmorf}.} until the symbol that changes between SL and TL is reached.

For example, to translate the Spanish word \textit{cama}, feminine noun, into the Catalan word \textit{llit}, masculine noun, the entry in the bilingual dictionary must be as shown in Figure \ref{fg:cama}. The gender must be specified (\emph{f}, \emph{m}) because, if not, the symbols for gender and number would be copied from the SL lexical form into de TL lexical form. Therefore, when translating from \texttt{es} to \texttt{ca}, we would obtain the lexical form \emph{llit} with the symbols \texttt{n f sg} or \texttt{n f pl}. In both cases, the generator would receive as input a word that is impossible to generate, since the Catalan morphological dictionary does not contain any entry with lemma \emph{llit} and feminine gender.

\begin{figure} \begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>cama<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="f"/></\textbf{l}> <\textbf{r}>llit<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{Bilingual dictionary entry for the translation \emph{cama} (\texttt{es})–\emph{llit} (\texttt{ca})} \label{fg:cama} \end{figure}

In this example, the number symbols are not specified; therefore, it works for the correspondence \textit{cama–llit} (singular) as well as for \textit{camas–llits} (plural). However, when there is a number change, the only way is to specify also the gender if the order used in all the dictionary for grammatical symbols is \emph{gender, number}.

By means of a direction restriction \texttt{r} we can indicate which translations are to be done only in one direction and not in the other one (see the description of the restrictions \texttt{LR} and \texttt{RL} in page \pageref{restric}). This is necessary when the correspondence between two lexical forms is not symmetrical; in such case, in the bilingual dictionary two or more entries have to be created and a direction restriction must be applied, like in the example shown in Figure~\ref{fg:postre}. In this example, when translating from Spanish to Catalan (\texttt{LR}), we must generate only plural forms, since the word \textit{postres} ("dessert" ) in Catalan does not have singular form. But, on the other hand, we will translate into Spanish only in plural form (although in Spanish the word has singular and plural forms), since it is not possible to determine, from the Catalan word, whether the number should be singular or plural.

\begin{figure}[htbp] \begin{small} \begin{alltt} <\textbf{e} \textsl{r}="LR"> <\textbf{p}> <\textbf{l}>postre<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="sg"/></\textbf{l}> <\textbf{r}>postres<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="pl"/></\textbf{r}> </\textbf{p}> </\textbf{e}>

<\textbf{e}> <\textbf{p}> <\textbf{l}>postre<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="pl"/></\textbf{l}> <\textbf{r}>postres<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="pl"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{Entries in the Spanish-Catalan bilingual dictionary for the correspondence \emph{postre}–\emph{postres} ("dessert")} \label{fg:postre} \end{figure}

\label{pg:GD} There is another problem due to grammatical divergences between two languages that is resolved with the help of two special symbols, \texttt{GD} (for \textit{gender to be determined}) and \texttt{ND} (for \textit{number to be determined}), symbols which have to be defined in the symbol section of the bilingual dictionary. This problem arises when the grammatical information of a SL lexical form is not enough to determine the gender (masculine or feminine) or the number (singular or plural) of the TL lexical form. Let’s put an example: the Spanish adjective \textit{común} ("common") is masculine and feminine at the same time (and, therefore, masculine–feminine, \texttt{mf}), but in Catalan the adjective has different forms for the masculine, \textit{comú}/\textit{comuns}, and the feminine, \textit{comuna}/\textit{comunes}. In the bilingual dictionary, the entry should be as shown in Figure~\ref{fg:comuna}: in the \texttt{LR} direction (from Spanish to Catalan), the gender information is not \texttt{m}, \texttt{f} nor \texttt{mf} but \texttt{GD}; this \textit{gender to be determined} will be determined next by the structural transfer module, by means of the application of the suitable transfer rules (usually, rules for the agreement between the lexical forms in a pattern; see Section \ref{ss:transfer} to obtain a detailed description of transfer rules). In an analogous way, a similar mechanism exists for singular–plural using the symbol \texttt{ND} (for example, in Spanish \textit{análisis} ("analysis") is singular and plural, whereas in Catalan the singular form is \textit{anàlisi} and the plural form \textit{anàlisis}).

\begin{figure}[htbp] \begin{small} \begin{alltt} <\textbf{e} \textsl{r}="LR"> <\textbf{p}> <\textbf{l}>común<\textbf{s} \textsl{n}="adj"/><\textbf{s} \textsl{n}="mf"/></\textbf{l}> <\textbf{r}>comú<\textbf{s} \textsl{n}="adj"/><\textbf{s} \textsl{n}="GD"/></\textbf{r}> </\textbf{p}> </\textbf{e}>

<\textbf{e} \textsl{r}="RL"> <\textbf{p}> <\textbf{l}>común<\textbf{s} \textsl{n}="adj"/><\textbf{s} \textsl{n}="mf"/></\textbf{l}> <\textbf{r}>comú<\textbf{s} \textsl{n}="adj"/><\textbf{s} \textsl{n}="m"/></\textbf{r}> </\textbf{p}> </\textbf{e}>

<\textbf{e} \textsl{r}="RL"> <\textbf{p}> <\textbf{l}>común<\textbf{s} \textsl{n}="adj"/><\textbf{s} \textsl{n}="mf"/></\textbf{l}> <\textbf{r}>comú<\textbf{s} \textsl{n}="adj"/><\textbf{s} \textsl{n}="f"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{Entries in the Spanish–Catalan bilingual dictionary for the correspondence \emph{común}–\emph{comú} ("common"), the first one for the translation from Spanish to Catalan and the two others for the translation from Catalan to Spanish} \label{fg:comuna}


\paragraph{Post-generation dictionaries} \label{ss:postgen}

In the morphological dictionary, the lexical forms which, once generated, may undergo contraction, apostrophation or other transformations, depending of which words are in contact with them in the output text, must have the post-generator activation mark (\texttt{<\textbf{a}/>}, see page \pageref{ss:a}) in the generation entry (\texttt{RL} direction). It is essential that the surface forms marked with the post-generator activation mark are identical in the morphological and the post-generation dictionaries of the same translator. In the post-generation dictionary, all entries begin with this activation mark.

In Figure~\ref{fg:postgen} there is an extract of the Spanish post-generator; the example shows how the contraction for \textit{de} and \textit{el} is done, to form the word \textit{del}. The paradigm \texttt{puntuación} not defined in the example contains the non-alphabetic characters that can appear in a text. We can see in the example that the entry for the preposition \emph{de} has the mark \texttt{<\textbf{a}/>}. The paradigm assigned to this entry, "\texttt{el}", is the one defined just above. According to this entry, when the system receives as input the left string of the entry (the part between \texttt{<\textbf{l}>}) concatenated to the left string of the paradigm (that is, when the input is \texttt{"}\texttt{<a/>\textbf{de}<b/>\textbf{el}<b/>"} or \texttt{"}\texttt{<a/>\textbf{de}\\<b/>\textbf{el}[puntuación]}\texttt{"}), the module delivers as output string (the part between \texttt{<r>} elements) the string \texttt{"}\textbf{del}\texttt{"} followed by the blanks represented with \texttt{<b/>} or by the symbols represented with \texttt{[puntu\-a\-ción]}. Note that, in the module output, all the marks \texttt{<\textbf{a}/>} have been removed.

\begin{figure}[htbp] \begin{small} \begin{alltt} <\textbf{dictionary}> <\textbf{pardefs}> ... <\textbf{pardef} \textsl{n}="el"> <\textbf{e}> <\textbf{p}> <\textbf{l}>el<\textbf{b}/></\textbf{l}> <\textbf{r}>l<\textbf{b}/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e}> <\textbf{p}> <\textbf{l}>el</\textbf{l}> <\textbf{r}>l</\textbf{r}> </\textbf{p}> <\textbf{par} \textsl{n}="puntuación"/> </\textbf{e}> </\textbf{pardef}> ... </\textbf{pardefs}> <\textbf{section} \textsl{id}="main" \textsl{type}="standard"> ... <\textbf{e}> <\textbf{p}> <\textbf{l}><\textbf{a}/>de<\textbf{b}/></\textbf{l}> <\textbf{r}>de</\textbf{r}> </\textbf{p}> <\textbf{par} \textsl{n}="el"/> </\textbf{e}> ... </\textbf{section}/> </\textbf{ditionary}> \end{alltt} \end{small} \caption{Post-generation dictionary data to perform the contraction for Spanish \emph{de} + \emph{el} = \emph{del} .} \label{fg:postgen} \end{figure} \nota{en l’exemple, "el" no ha de portar la marca d’activació oi? - l’he treta de l’exemple, treure-la dels diccionaris (Mikel?)}


\subsubsection{Multiword lexical units} \label{ss:multipalabras}

The designed dictionary format allows the creation of \textit{multiword lexical units} —in short, \textit{multiwords}— of different kinds, depending on the problem to be approached.

In this project we have considered three basic types of multiwords: \begin{enumerate} \item The most simple case are \textit{multiwords without inflection}, which consist of only one lexical form: the lemma is made of two or more invariable orthographic words but it is tagged as a unit. Figure \ref{fig:msf} shows an example of invariable multiword (the Spanish expression \emph{hoy en día}, "nowadays"): It is made of three words separated by a blank (\texttt{<\textbf{b}/>}) and, although it actually consists of an adverb, a preposition and a noun, it is tagged as an adverb as a whole, since it acts as one.

\begin{figure} \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="hoy en día"> <\textbf{p}> <\textbf{l}>hoy<\textbf{b}/>en<\textbf{b}/>día</\textbf{l}> <\textbf{r}>hoy<\textbf{b}/>en<\textbf{b}/>día<\textbf{s} \textsl{n}="adv"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{Example of multiword without inflection in the morphological dictionary} \label{fig:msf} \end{figure}

\item A more complicated issue is the case of \textit{compound multiwords}, made of more than one lexical form, each one with its grammatical symbols. The words they are made of are considered not to build a semantic unit like in the previous case, but to appear together building a unit due to contact reasons (phonetic or orthographic reasons). In this category we include \textit{contractions} and \textit{enclitic pronouns} accompanying verbs. To mark this phenomenon we use the tag \texttt{<\textbf{j}>} described in page~\pageref{ss:j}. You can see an example in Figure~\ref{fig:cont}, in which the analysis of \emph{del} delivers a lexical multiform made of two lexical forms: \emph{de}, preposition, and \emph{el}, singular masculine definite determiner, linked with the \texttt{<\textbf{j}/>} element. The analyser and the part-of-speech tagger handle this multiwords as a unit; however, before entering the transfer module, they are processed by an auxiliary module called \texttt{pretransfer} (see section \ref{se:pretransfer}) which is responsible for separating the lexical forms they are made of. This way, they reach the transfer module as independent forms; the linguist has to decide whether they have to be joined again (which must be done in the structural transfer module) or they have to remain as independent forms through the next modules.

In our system, the elements forming a contraction continue as independent forms, and the post-generator is responsible for making the contractions in the target language if it is necessary. On the other hand, enclitic pronouns are joined again to the verb by means of a structural transfer rule (see Section \ref{ss:transfer}), so the verb plus its enclitic pronouns get into the generation module as a single lexical multiform, its components joined with a \texttt{<\textbf{j}/>}. Therefore, entries containing enclitic pronouns must not have any direction restriction, as can be seen in the example in Figure \ref{fig:encl}, which shows a part of the paradigm for the Spanish verb "dar" ("to give"), specifically the entry for the infinitive form joined to an enclitic pronoun.

\begin{figure} \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="del" \textsl{r}="LR"> <\textbf{p}> <\textbf{l}>del</\textbf{l}> <\textbf{r}>de<\textbf{s} \textsl{n}="pr"/><\textbf{j}/> el<\textbf{s} \textsl{n}="det"/><\textbf{s} \textsl{n}="def"/> <\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="sg"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{Entry in the morphological dictionary for the analysis of a contraction (the Spanish contraction \emph{del})} \label{fig:cont} \end{figure}

\begin{figure} \begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>ar</\textbf{l}> <\textbf{r}>ar<\textbf{s} \textsl{n}="vblex"/><\textbf{s} \textsl{n}="inf"/><\textbf{j}/></\textbf{r}> </\textbf{p}> <\textbf{par} \textsl{n}="S__cantar"/> </\textbf{e}> \end{alltt} \end{small} \caption{A fragment of the inflection paradigm for the Spanish verb \emph{dar} ("to give"), which shows the entry for the infinitive form followed by an enclitic pronoun. Enclitic pronouns are contained in the paradigm \texttt{S\_\_cantar}. Note that, unlike in Figure \ref{fig:cont}, this entry is both for analysis and generation.} \label{fig:encl} \end{figure}

\item The most complicated case in our system is the case of \textit{multiwords with inner inflection} inside the lemma (or "split lemma" forms), like the example shown in Figure \ref{fig:echardemenos}. The lemma of this kind of multiwords has one part with inflection (the \emph{lemma head}) followed by one invariable part (the \emph{lemma tail}). The invariable part has to be put between \texttt{<\textbf{g}>} elements, so that it can be moved to the position immediately after the lemma head to obtain the whole lemma of the multiword. For example, the lemma of the Spanish multiwords \emph{echó de menos} ("he/she missed"), \emph{echándole de menos} ("missing him/her"), etc. has to be \emph{echar de menos} ("to miss"), since this form will be the one searched in the bilingual dictionary to find its translation. This means that the invariable lemma tail (\emph{de menos}) has to be moved after the uninflected lemma head (\emph{echar}). This moving backwards will be done by the auxiliary module \texttt{pretransfer} (see section \ref{se:pretransfer}) which runs before the structural transfer module.

To understand the example in Figure \ref{fig:echardemenos}, you have to be aware that the paradigm defining the verb \emph{echar} includes, besides the verb inflection, the enclitic pronouns that can appear at the end of the inflected forms of the verb; in the output lexical multiform, this enclitic pronouns are joined using the empty element \texttt{<\textbf{j}/>}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="echar de menos"> <\textbf{i}>ech</\textbf{i}> <\textbf{par} \textsl{n}="aspir/ar__vblex"/> <!-it includes enclitic pronouns –> <\textbf{p}> <\textbf{l}><\textbf{b}/>de<\textbf{b}/>menos</\textbf{l}> <\textbf{r}><\textbf{g}><\textbf{b}/>de<\textbf{b}/>menos</\textbf{g}></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{A morphological dictionary entry containing a \texttt{<\textbf{g}>} group.} \label{fig:echardemenos} \label{fig:hacertilin} \end{figure}

When the translation is also a \emph{split lemma} (for example, the translation of "to miss" in Catalan is \emph{trobar a faltar}, with forms like \emph{trobem a faltar}, \emph{trobar-lo a faltar}, etc.), it is necessary to place again the lemma tail in its original place, after the inflected form plus the enclitic pronouns (if any), and indicate the correspondence of these invariable parts of the lemma (\emph{de menos}, \emph{a faltar}) at both sides of the translation. So, in the example of Figure ~\ref{fig:echardemenos}, the \texttt{<\textbf{g}>} element is used to mark the group ‘\texttt{<b/>de<b/>menos}’ in the morphological dictionary, whereas in the bilingual dictionary (see Figure~\ref{fig:menosfaltar}), the \texttt{<\textbf{g}>} element is used to establish the correspondence between the groups “\texttt{<b/>de<b/>menos}” and “\texttt{<b/>a<b/>faltar}”. \nota{I com serà el cas de “dirección general” - “direcciones generales”?}

If the translation is not a \emph{split lemma}, you do not need to insert any \texttt{<\textbf{g}>} element in the target language string.


\begin{figure} \begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>echar<\textbf{g}><\textbf{b}/>de<\textbf{b}/>menos</\textbf{g}><\textbf{s} \textsl{n}="vblex"/></\textbf{l}> <\textbf{r}>trobar<\textbf{g}><\textbf{b}/>a<\textbf{b}/>faltar</\textbf{g}><\textbf{s} \textsl{n}="vblex"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \caption{A bilingual dictionary entry containing two corresponding \texttt{<\textbf{g}>} groups.} \label{fig:menosfaltar} \end{figure}

\subsubsection{Metaparadigms} \label{ss:metaparadigmas}

\nota{Marco diu: Especificar la DTD?}

When developing the dictionaries for the Occitan translator, we were faced with a new need: we wanted to be able to specify paradigms for verbs that had a same inflection pattern but whose root changed in the different inflected forms. With the existing paradigm system, a new paradigm had to be created for each of these verbs, since it was only possible to specify an inflection regularity pattern for a group of verbs with invariable root. With metaparadigms, it is possible to specify the inflection regularity as well as verb root variations.

At the same time, metaparadigms allow the specification, in a single paradigm, of variations in the grammatical symbols of a lemma. That is, several lemmas can refer to a same metaparadigm even if they have different grammatical symbols. Whereas for Occitan, metaparadigms have allowed having a same paradigm for entries with root variations, for English, these have allowed having a same paradigm for entries with variations in their grammatical symbols.

Related with this, we created the concept of metadictionary: it is a dictionary which contains metaparadigms as well as the normal paradigms used so far. The name of a metadictionary is \texttt{apertium-PAIR.}$L_1$\texttt{.metadix} (for example, for the English monolingual dictionary in the Apertium-en-ca system, \texttt{apertium-en-ca.en.metadix}). When linguistic data are compiled these dictionaries are pre-processed, so that they have the appropriate format for the dictionary compiler.

\paragraph{Specification of metaparadigms}

Metaparadigms are defined in the \texttt{<\textbf{pardefs}>} section of the monolingual dictionary, the same section where also the rest of the dictionary paradigms are defined. A metaparadigm, just like a paradigm, has a name specified in the attribute \texttt{n}. This name will have the same characteristics as in the other paradigms, with the difference that the variable part of the lemma root will be in brackets and in capital letters, as you can see in this example:

\begin{alltt} <\textbf{pardef} n="m/é[T]er\_\_vblex"> \end{alltt}

This is the definition of a verb paradigm, where the inflection endings have a variable part in the root. The inflection paradigms specified inside this metaparadigm have to present inflection only in the part at the right of the brackets, for example like the one specified in the paradigm:

\begin{alltt} <\textbf{par} n="mét/er\_\_vblex"/> \end{alltt}

In conclusion, a complete example of metaparadigm definition would be:

\begin{alltt} <\textbf{pardef} n="m/é[T]er__vblex"> <\textbf{e}> <\textbf{p}> <\textbf{l}>e</\textbf{l}> <\textbf{r}>é</\textbf{r}> </\textbf{p}> <\textbf{i}><prm/></\textbf{i}> <\textbf{par} n="sent/eria__vblex"/> </\textbf{e}> <\textbf{e}> <\textbf{i}>é<prm/></\textbf{i}> <\textbf{par} n="mét/er__vblex"/> </\textbf{e}> </\textbf{pardef}>


The tag \texttt{<\textbf{prm}/>} is the marker that is used to place the variable text part (the root variation) in the paradigm definition.

Once a metaparadigm is defined, we may want that a verb uses it. To do so, in the verb entry (inside a \texttt{<\textbf{e}>} element) we must indicate the suitable metaparadigm and, through the attribute \texttt{prm}, define with which letters we want to replace the variable part specified in brackets. For example:

\begin{alltt} <\textbf{e} lm="acuélher"> <\textbf{i}>acu</\textbf{i}> <\textbf{par} n="m/é[T]er__vblex" prm="lh"/> </\textbf{e}>


This entry defines the Occitan verb \emph{acuélher} ("to receive") and specifies that its inflection paradigm is the one defined by the metaparadigm \texttt{m/é[T]er\_\_vblex}, but replacing \texttt{T} with \texttt{lh}; that is, the letters following \emph{acu} will be \emph{élher} instead of \emph{éter}.

As mentioned before, metaparadigms can also be used for entries which have some variation in their grammatical symbols. The way to specify them is basically the same: the variable part must be specified in the entry with the attribute \texttt{sa}, whereas in the paradigm the tag \texttt{<\textbf{sa}>} has to be placed where the optional grammatical symbol should appear.

For example, we have the following metaparadigm:

\begin{alltt} <\textbf{pardef} n="house__n"> <\textbf{e}> <\textbf{p}> <\textbf{l}/> <\textbf{r}><\textbf{s} n="n"/><sa/><\textbf{s} n="sg"/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e}> <\textbf{p}> <\textbf{l}>s</\textbf{l}> <\textbf{r}><\textbf{s} n="n"/><sa/><\textbf{s} n="pl"/></r> </\textbf{p}> </\textbf{e}> </\textbf{pardef}>


and the following entry:

\begin{alltt} <\textbf{e} lm="time"> <\textbf{i}>time</\textbf{i}> <\textbf{par} n="house__n" sa="unc"/> </\textbf{e}> \end{alltt}

where \emph{unc} means that the noun is uncountable.

In the metaparadigm, the tag \texttt{<\textbf{sa}>} shows the place where the grammatical symbol is to be placed if an entry contains the attribute \texttt{sa} with a value, as happens in the entry for \emph{time}.

A dictionary which contains entries like the ones described here is called metadictionary and must be pre-processed in order to generate a dictionary that follows the DTD for Apertium 2, since the engine does not allow the direct use of metaparadigms. The next section describes how is this pre-processing like.

\paragraph{Pre-processing of the metadictionary}

A metadictionary is an XML file to which two XSLT style sheets are applied, in order to pre-process the metaparadigms and obtain a dictionary with all the paradigms derived from the metaparadigms. The first style sheet, \texttt{buscaPar.xsl}, produces the list of verbs that use metaparadigms and deletes the possible repetitions of metaparadigms to be expanded. This style sheet generates, in combination with the sheet \texttt{principal.xsl}, a second style sheet called \texttt{gen.xsl}, which processes the metadictionary with the list of metaparadigms to be expanded and generates a dictionary in Apertium 2 format. Basically, what this generated style sheet does is:


\item In verb entries, if a verb uses a metaparadigm, this metaparadigm is replaced by the corresponding expanded and deparametrized paradigm. Thus, the previous example entry:

\begin{alltt} <\textbf{e} lm="acuélher"> <\textbf{i}>acu</\textbf{i}> <\textbf{par} n="m/é[T]er__vblex" prm="lh"/> </\textbf{e}> \end{alltt}

would be deparametrized and expanded into:

\begin{alltt} <\textbf{e} lm="acuélher"> <\textbf{i}>acu</\textbf{i}> <\textbf{par} n="m/élher__vblex"/> </\textbf{e}> \end{alltt}

\item On the other hand, since from the first pass the system knows which paradigms have to be created from metaparadigms, these are created. In the previous example, from the metaparadigm:

\begin{alltt} <\textbf{pardef} n="m/é[T]er__vblex"> <\textbf{e}> <\textbf{p}> <\textbf{l}>e</\textbf{l}> <\textbf{r}>é</\textbf{r}> </\textbf{p}> <\textbf{i}><prm/></\textbf{i}> <\textbf{par} n="sent/eria__vblex"/> </\textbf{e}> <\textbf{e}> <\textbf{i}>é<prm/></\textbf{i}> <\textbf{par} n="mét/er__vblex"/> </\textbf{e}> </\textbf{pardef}> \end{alltt}

the system would generate the paradigm \texttt{"m/élher\_\_vblex"} :

\begin{alltt} <\textbf{pardef} n="m/élher__vblex"> <\textbf{e}> <\textbf{p}> <\textbf{l}>e</\textbf{l}> <\textbf{r}>é</\textbf{r}> </\textbf{p}> <\textbf{i}>lh/></\textbf{i}> <\textbf{par} n="sent/eria__vblex"/> </\textbf{e}> <\textbf{e}> <\textbf{i}>élh</\textbf{i}> <\textbf{par} n="mét/er__vblex"/> </\textbf{e}> </\textbf{pardef}> \end{alltt}


After the metadictionary has been processed according to these steps, a .dix dictionary is generated which follows the DTD for Apertium 2 and which can already be compiled.

In the case of our second example, where the variable part was the sequence of grammatical symbols in the paradigm, the style sheets would be applied and, from the value \emph{unc} specified in the attribute \texttt{sa}, the following paradigm would be generated:

\begin{alltt} <\textbf{pardef} n="house__n__unc"> <\textbf{e}> <\textbf{p}> <\textbf{l}/> <\textbf{r}><\textbf{s} n="n"/><\textbf{s} n="unc"/><\textbf{s} n="sg"/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e}> <\textbf{p}> <\textbf{l}>s</\textbf{l}> <\textbf{r}><\textbf{s} n="n"/><\textbf{s} n="unc"/><\textbf{s} n="pl"/></r> </\textbf{p}> </\textbf{e}> </\textbf{pardef}>


for nouns the morphological analysis of which should be (in data stream format):

\begin{alltt} time<n><unc><sg> \end{alltt}

In this case, metaparadigms allows the use of the same paradigm for entries with the same inflection but with a slightly different morphological analysis.

It is important to note that, when a dictionary uses metaparadigms and, accordingly, its name has the extension \texttt{.metadix}, this will be the file where dictionary changes have to be made (adding, changing or deleting entries or paradigms), since the file \texttt{.dix} is automatically generated from this one every time linguistic data are compiled and, therefore, any changes made in the latter will be overwritten during compilation.

\subsubsection{Analysis characters} \label{acx}

Version 3 of the Apertium platform includes Unicode support; however, this lead to a new problem: alternate characters. Unicode supports several character sets, which include several characters that look identical or almost identical, but which have a different numeric value.

As a solution, equivalent characters can be specified in a file that complements the morphological dictionary. As the morphological dictionary is compiled, whenever a character mentioned in the analysis character specification is encountered, its equivalents are included as though they had been specified using entries specified with the \texttt{LR} restriction within the dictionary.

\begin{figure} \begin{small} \begin{alltt} <?\textbf{xml} \textsl{version}="1.0"?> <\textbf{analysis-chars}> <\textbf{char} \textsl{value}="’"> <\textbf{equiv-char} \textsl{value}="\&#x2019;"/> <\textbf{equiv-char} \textsl{value}="\&#x2BC;"/> </\textbf{char}> <\textbf{char} \textsl{value}="\&#183;"> <\textbf{equiv-char} \textsl{value}="."/> </\textbf{char}> </\textbf{analysis-chars}> \end{alltt} \end{small} \caption{Analysis character specification file} \label{fg:acxsample} \end{figure}

A sample analysis characters specification file can be seen in Figure \ref{fg:acxsample}. It’s worth noting that the analysis characters file can only be used when there is a 1:1 mapping between individual characters; in the case of multiple characters, it would be better to use the example given earlier, in Figure \ref{fig:pardef2}

\subsection[Automatic generation of the modules]{Automatic generation of the lexical processing modules} \label{se:compiladoresdic}

The four lexical processing modules (morphological analyser, lexical transfer, morphological generator and post-generator) are compiled from dictionaries by means of a single compiler based on letter transducers \cite{roche97}. This compiler is much faster than the ones used in the systems \textsf{interNOSTRUM} \cite{canals01b,garridoalenda01p,garrido99j} and \textsf{Traductor Universia} \cite{garrido03p, gilabert03j}, thanks to the use of new compiler building strategies and the minimization of partial transducers during the building process \cite{ortiz05j}.

The division of dictionary entries into lemma and paradigm enables the effective construction of minimal letter transducers. The compiler makes the most of the factorization allowed by paradigms in order to speed up the construction. Taking into account that, in most European languages, word variations occur at the end or the beginning of words, we took advantage of this fact to improve the construction speed of the minimal transducer.

Paradigms are also minimized before being inserted in the big transducer in order to reduce the size of the big transducer before its minimization. Since, before minimizing, the paradigms of the dictionaries for the languages we have dealt with usually have just a few hundreds of states, the minimization of these paradigms is a very fast process.

If we assume that an entry can have at any point a reference to a paradigm, we could decide to copy at this point the transducer calculated in the paradigm definition. The method used in \emph{Apertium} is based on the idea that it is not always necessary to copy, because in certain cases it is possible to reuse a paradigm that was already copied. In particular, two or more entries that share a paradigm as a suffix can reuse the same copy of this paradigm; the same can be said when it is as a prefix. However, generally it is not possible to reuse paradigms if they are located in intermediate positions of different entries, since new suffixes (or prefixes) can be added to existing entries, which causes the information inserted in the transducer not to be consistent with the dictionary, and therefore the generated transducer would be incorrect (it would add string pairs that are not present in the formal language defined by dictionaries).

Minimal letter transducers are built as explained next. From a string transduction it is possible to build a \textit{sequence of letter transductions} $S(s:t)$ with length $N = \max(|s|,|t|)$ which is defined as follows for each element $1 \leq i \leq N$:

\begin{equation} \label{eq:transletras} S_i(s:t)=\left\{ \begin{array}{ll} (s_i:\theta) & \textrm{if } i \leq |s| \wedge i > |t| \\ (\theta:t_i) & \textrm{if } i \leq |t| \wedge i > |s| \\ (s_i:t_i) & \textrm{in other cases} \end{array}\right. \label{e:montaje} \end{equation}

It should be emphasized that the construction design forbids the existence of a $(s:t)$ that is equal to $(\epsilon:\epsilon)$, which is crucial for the consistence of the building method.

The building method uses two procedures: the \textit{assembly} procedure inferred from equation \ref{e:montaje}, and the minimization procedure, which is executed by a conventional minimization algorithm \cite{vandesnepscheut93b} for deterministic finite state automata, which consists of inverting, determining, inverting again and determining again, taking as the alphabet of the automaton to be minimized the Cartesian product of $L$ and as empty transition the $\left(\theta:\theta\right)$.

\begin{figure} \begin{center} \includegraphics[width=10cm]{fig1} \end{center} \caption{Building of the dictionary as prefix acceptor and link to paradigms through transitions $\left(\theta:\theta\right)$.} \label{fig:construccion} \end{figure}

\begin{figure} \begin{center} \includegraphics[width=8cm]{fig2} \end{center} \caption{Minimized paradigm "-es \textbf{n m}" used in Figure \ref{fig:construccion}.} \label{fig:paradigmapan} \end{figure}

\begin{figure} \begin{center} \includegraphics[width=8cm]{fig3} \end{center} \caption{Minimized paradigm "z/-ces \textbf{n m}" used in Figure \ref{fig:construccion}.} \label{fig:paradigmavez} \end{figure}

Figure \ref{fig:construccion} shows a simplified example of the assembly process. Transductions, composed as in the equation \ref{e:montaje}, are inserted one by one in a transducer in the form of a \textit{prefix acceptor} or \textit{trie}, that is, in a way that there is only one node for each common prefix of the group of transductions that form the dictionary. With the suffixes of the transductions (that are not shared) new states are created. In the point where there is a reference to a paradigm, a replica of this paradigm is created and a link is created to the dictionary entry which is being inserted in the transducer by means of a null transduction $\left(\theta:\theta\right)$.

Each paradigm, as it can be seen as a little dictionary, has been built according to this same procedure and been minimized to reduce the size of the content when building the big dictionary. In Figures \ref{fig:paradigmapan} and \ref{fig:paradigmavez} you can see the state of the paradigms used in Figure \ref{fig:construccion} after its minimization.

\section{Part-of-speech tagger} \label{ss:tagger}

\subsection{Module description } \label{functagger}

The part-of-speech tagger is based on first-order hidden Markov models~\cite{rabiner89}, that is, on statistical data. The states of the Markov model represent parts of speech, and the observable parameters are ambiguity classes~\cite{cutting92a}, formed by groups of parts of speech.

In spite of working with statistical information, the training and behaviour of the tagger improve with the application of restrictions that forbid certain sequences of parts of speech (in the first-order models, these sequences can only include two parts of speech). For example, in Spanish or Catalan a preposition can never be followed by a verb in personal form; this restriction is of great help when the word after a preposition is ambiguous and one of its possible analyses is a verb in personal form (e.g., \emph{de trabajo}, \emph{en libertad}, etc.). Restrictions are explicitly declared in the tagger definition file, sometimes in the form of \emph{prohibitions} and sometimes of \emph{obligations}.

The morphological tags which the tagger works with are not the same as the ones used in the morphological analyser. Usually, the information delivered by the analyser is too detailed for the purposes of the part-of-speech disambiguation (for example, for most purposes, it suffices to group in the same category all common nouns, regardless of their gender and number). The use of finer-grained tags does not improve the results, whereas it increases the number of parameters to be estimated and intensifies the problem of lack of linguistic resources such as manually disambiguated texts. For this reason, in the tagger file one has to specify how to group the \emph{fine-grained} tags delivered by the morphological analyser into more general \emph{coarse} tags —which we will call \emph{categories}— that will be used in the part-of-speech disambiguation. Apart from coarse categories, one can also define lexicalized tags. Basically there are two types of lexicalizations described in bibliography: one type adds new observables and the other one, in addition, adds new states to the Markov model~\cite{pla04}; the tagger in Apertium uses the latter lexicalization type.

It is important to note that, in spite of working with \emph{coarse} categories, the tagger outputs fine-grained tags like the ones from the morphological analyser. Sometimes it may occur that the morphological analyser delivers, for a certain word, two or more fine-grained tags that can be grouped under the same tagger category: e.g. in Spanish \emph{cante} can be the 1st or the 3rd singular person of the subjunctive present of the verb \emph{cantar} ("to sing"); both fine-grained tags, \texttt{\emph{<vblex><prs><p1><sg>}} and \texttt{\emph{<vblex><prs><p3><sg>}}, are grouped under the tagger category \ \texttt{VLEXSUBJ} (\emph{subjunctive verb}). In this case, one of both fine tags is discarded; in the tagger definition file it is possible to define which fine-grained tag, among the ones that compose a coarse tag, will be delivered after disambiguation.

\subsection{Data for the part-of-speech tagger} \label{datostagger} \subsubsection{Introduction} \label{ss:introtagger} We describe next the format of the files that specify how to group the \emph{fine-grained} tags delivered by the morphological analyser into more general \emph{coarse} tags. In this files, moreover, one can specify \emph{restrictions} that help in the estimation of the statistical model underlying the process of lexical disambiguation, as well as preference rules to be applied when two fine-grained tags belong to the same category.

The tagger assumes that, in the input stream, lexical forms will be appropriately delimited, as described in the format specification for the data stream between modules (Section \ref{se:flujodatos}). In brief, the format of the data delivered by the morphological analyser is the following: \begin{equation} \label{eq:formaanalizada} \begin{array}{rcl} \mbox{analysedform}&\to& \mbox{lexicalmultiform}\; [\; \mbox{lexicalmultiform} \; ]^* \\ \mbox{lexicalmultiform}&\to& \mbox{lexicalform}\; [\;\mbox{lexicalform}\; ]^*\;\mbox{lemma-queue?} \\ \mbox{lexicalform}&\to&\mbox{lemma}\;\mbox{finetag}\\ \mbox{lemma-queue}&\to&\mbox{lemma}\\ \mbox{finetag}&\to&\mbox{morphsymbol}\;[\;\mbox{morphsymbol}\;]^* \\ \end{array} \end{equation} \label{formaanalizada}


\begin{itemize} \item \emph{analysedform} is all the information delivered for each surface form in the output of the morphological analyser \item \emph{lexicalmultiform} is a sequence of one or more lexical forms followed, optionally, by an invariable queue as happens in some multiwords (like the Spanish expression \emph{cántale las cuarenta}). \item \emph{lexicalforms}\footnote{Separated from each other by a delimiter which corresponds to the \texttt{<j/>} element (see page \pageref{ss:j}).} are units made of one lemma and one or more grammatical symbols (which compose the fine-grained tag) with the output information of the analyser \item \emph{lemma-queue} is made of one or more lemmas \footnote{Separated from each other by the \texttt{<b/>} element (see page~\pageref{s3:b}).} that are the invariable part of a multiword. The queue of a multiword is made of the lemma or lemmas with no inflection that follow the lemmas with inflection. For example, the Spanish multiword \emph{cantar las cuarenta} ("to lecture", "to reproach") can take the forms \emph{cántale las cuarenta}, \emph{(le) cantaré las cuarenta}, \emph{cantándole las cuarenta}, etc. In this case, the queue would be \emph{las cuarenta} (see page~\pageref{ss:multipalabras} for more information).

\item \emph{finetag} is made of one or more grammatical symbols (\emph{símbologram}). \end{itemize}

For example, the entry for the Spanish ambiguous surface form \emph{correos} would have two lexical multiforms; the first lexical multiform would have one single lexical form, with lemma \emph{correo} ("post office") and a fine tag made of the grammatical symbols \emph{common noun}, \emph{masculine}, \emph{plural}; the second lexical multiform would be a sequence of two lexical forms, one with lemma \emph{correr} ("to move") and a fine tag made of the grammatical symbols \emph{lexical verb}, \emph{imperative}, \emph{second person}, \emph{plural}, and the other one with lemma \emph{vosotros} ("you") and fine tag made of the grammatical symbols \emph{pronoun}, \emph{enclitic}, \emph{second person}, \emph{masculine-feminine}, \emph{plural}.

\nota{An explanation of how a word containing more than one lexical form is treated when no multilexical form is defined for it should be added}

\subsubsection{Format specification} \label{formatotagger} The format of the file (encoded in XML) is specified by the DTD that can be found in Appendix~\ref{ss:DTD_desambiguador}.

The meaning of the different tags is the following: \begin{description} \item[\texttt{tagger}]: is the root element; its mandatory attribute \texttt{name} is used to specify the name of the tagger generated from the file. \item[\texttt{tagset}]: defines the \emph{coarse} tagset or categories with which the tagger works. Categories are defined by the fine-grained tags output by the morphological analyser. \item[\texttt{def-label}]: defines a category or coarse tag (whose name is specified in the mandatory attribute \texttt{name}) by means of a list of fine tags defined with one or more \texttt{tags-item} elements; an optional attribute \texttt{closed} indicates whether this is a closed category; if this is the case, it is assumed that an unknown word can never belong to this category.\footnote{Closed categories are those that do not grow when new words are created: prepositions, determiners, conjunctions, etc.}

The more specific categories \emph{must} be defined before the more general ones. When the definition of a general category implicitly includes that of a specific category defined before, it is understood that it refers to all cases \emph{except} the ones defined by the more specific category.

\item[\texttt{tags-item}]: is used to define a fine-grained tag by means of a sequence of grammatical symbols. The sequence of grammatical symbols that make up the fine tag is specified in the mandatory attribute \texttt{tags}. In this sequence, symbols are separated by a dot, and the asterisk “\texttt{*}” is used to express that any sequence of symbols may appear in its place. It is also possible to define lexicalized categories, specifying the lemma of the word in the attribute \texttt{lemma}.

\item[\texttt{def-mult}]: defines special categories (\emph{multicategories}) made of more than one category, in order to deal with entries with more than one lexical form, like in the example given in the previous section. Each category is defined as a set of valid sequences (\texttt{sequence}) of previously defined categories or of fine-grained tags. It is designed for contractions, verbs with enclitic pronouns, etc.

\item[\texttt{sequence}]: defines a sequence of elements, which can be categories (\texttt{label-item}) or fine-grained tags (\texttt{tags-item}). Using fine-grained tags directly is useful if one wishes to use a sequence of grammatical symbols that is not part of any previously defined fine tag \nota{MG: en comptes de ’fine tag’ no es refereix aquí a ’category’?} or that represents a greater specialization of a defined fine tag \nota{ídem: category}.

\item[\texttt{label-item}]: is used to refer to a category or coarse tag previously defined, to be specified in the mandatory attribute \texttt{label}.

\item[\texttt{forbid}]: this (optional) section is aimed to define restrictions as sequences of categories \texttt{label-sequence} that can not occur in the language involved. In the current version, due to the fact that the tagger is based on first-order hidden Markov models, sequences can only be made of \emph{two} \texttt{label-items}.

\item[\texttt{label-sequence}]: defines a sequence of categories (\texttt{label-item}).

\item[\texttt{enforce-rules}]: this (optional) section allows defining restrictions in the form of obligations.

\item[\texttt{enforce-after}]: defines a restriction that forces that a certain category can only be followed by the categories belonging to the set of categories defined in \texttt{label-set}. Note that this kind of restrictions is equivalent to defining several forbidden (\texttt{forbid}) sequences (\texttt{label-sequence}) with the category defined in the mandatory attribute \texttt{label} and the rest of categories that do not belong to the set defined in \texttt{label-set}. For this reason, this kind of restriction must be used very cautiously.

\item[\texttt{label-set}]: defines a set of categories (\texttt{label-items}).

\item[\texttt{preferences}]: used to define priorities in terms of which fine-grained tag must be delivered in the tagger output when two or more fine tags are assigned to the same category.

\item[\texttt{prefer}]: specifies that, in case of conflict between different fine-grained tags assigned to the same category, the tagger must output the tag specified in the mandatory attribute \texttt{tags}. If a category contains more than one of the fine tags included in these \texttt{prefer} elements, the tag defined in the first place will be the selected one. \end{description}

Figures~\ref{fg:exemple_desambiguador1} and~\ref{fg:exemple_desambiguador2} contain an example with the most significant parts of a tagger specification file defined by the DTD just described.

% DTD moguda a Apèndix

\begin{figure}[htbp] \begin{small} \begin{alltt} <?\textsl{xml} \textsl{version}="1.0" \textsl{encoding}="iso-8859-1"?> <!\textsl{DOCTYPE} \textbf{tagger} SYSTEM "tagger.dtd"> <\textbf{tagger} \emph{name}="es-ca"> <\textbf{tagset}> <\textbf{def-label} \textsl{name}="adv"> <\textbf{tags-item} \textsl{tags}="adv"/> </\textbf{def-label}> <\textbf{def-label} \textsl{name}="detnt" \textsl{closed}="true"> <\textbf{tags-item} \textsl{tags}="detnt"/> </\textbf{def-label}> <\textbf{def-label} \textsl{name}="detm" \textsl{closed}="true"> <\textbf{tags-item} \textsl{tags}="det.*.m"/> </\textbf{def-label}> <\textbf{def-label} \textsl{name}="vlexpfci"> <\textbf{tags-item} \textsl{tags}="vblex.pri"/> <\textbf{tags-item} \textsl{tags}="vblex.fti"/> <\textbf{tags-item} \textsl{tags}="vblex.cni"/> </\textbf{def-label}> <\textbf{def-mult} \textsl{name}="infserprnenc" \textsl{closed}="true"> <\textbf{sequence}> <\textbf{label-item} \textsl{label}="vserinf"/> <\textbf{label-item} \textsl{label}="prnenc"/> </\textbf{sequence}> <\textbf{sequence}> <\textbf{label-item} \textsl{label}="vserinf"/> <\textbf{label-item} \textsl{label}="prnenc"/> <\textbf{label-item} \textsl{label}="prnenc"/> </\textbf{sequence}> </\textbf{def-mult}> <\textbf{def-mult} \textsl{name}="prepdet" \textsl{closed}="true"> <\textbf{sequence}> <\textbf{label-item} \textsl{label}="prep"/> <\textbf{tags-item} \textsl{tags}="det.def.m.sg"/> </\textbf{sequence}> </\textbf{def-mult}> </\textbf{tagset}> <!– ... –> \end{alltt} \end{small} \caption{Example of a tagger definition file (continues in Figure~\ref{fg:exemple_desambiguador2}).} \label{fg:exemple_desambiguador1} \end{figure}

\begin{figure}[htbp] \begin{small} \begin{alltt} <!– ... –> <\textbf{forbid}> <\textbf{label-sequence}> <\textbf{label-item} \textsl{label}=="prep"/> <\textbf{label-item} \textsl{label}=="vlexpfci"/> </\textbf{label-sequence}> <!– ... –> </\textbf{forbid}> <\textbf{enforce-rules}> <\textbf{enforce-after} \textsl{label}=="prnpro"> <\textbf{label-set}> <\textbf{label-item} \textsl{label}=="prnpro"/> <\textbf{label-item} \textsl{label}=="vlexpfci"/> <!– ... –> </\textbf{label-set}> </\textbf{enforce-after}> <!– ... –> </\textbf{enforce-rules}> <\textbf{preferences}> <\textbf{prefer} \textsl{tags}="vblex.pii.p3.sg"/> <\textbf{prefer} \textsl{tags}="vbser.pii.p3.sg"/> <!– ... –> </\textbf{preferences}> </\textbf{tagger}> \end{alltt} \end{small} \caption{Example of a tagger definition file (comes from Figure~\ref{fg:exemple_desambiguador1}).} \label{fg:exemple_desambiguador2} \end{figure}

\subsection{Some questions about the training of the part-of-speech tagger} The training of the part-of-speech tagger can be made both in a supervised manner, using manually disambiguated texts, and a unsupervised manner, using ambiguous texts.

When the training is made with ambiguous texts (unsupervised), the format of the required text can be automatically obtained from a plain text corpus in the chosen language using the system’s morphological analyser; in this case, the format of the text forms will be like the one defined in the figure~\ref{eq:formaanalizada2} (its description can be found in page~\pageref{formaanalizada}). As the chart shows, each analysed surface form can have more than one analysis (an \emph{analysedform} can give as a result more than one \emph{lexicalmultiform}).

\begin{equation} \label{eq:formaanalizada2} \begin{array}{rcl} \mbox{analysedform}&\to& \mbox{lexicalmultiform}\; [\; \mbox{lexicalmultiform} \; ]^* \\ \mbox{lexicalmultiform}&\to& \mbox{lexicalform}\; [\;\mbox{lexicalform}\; ]^*\;\mbox{lemma-queue?} \\ \mbox{lexicalform}&\to&\mbox{lemma}\;\mbox{finetag}\\ \mbox{lemma-queue}&\to&\mbox{lemma}\\ \mbox{finetag}&\to&\mbox{morphsymbol}\;[\;\mbox{morphsymbol}\;]^* \\ \end{array} \end{equation} \label{formaanalizada2}

For the supervised training we need manually disambiguated text. The format of the text forms in this case will be like the format delivered by the morphological analyser (see Section~\ref{se:flujodatos}) except that, being the text already disambiguated, a surface form can never produce more than one lexical form, as shown in Figure~\ref{eq:formadesambiguada} (a \emph{disambiguatedform} will consist always of a single \emph{lexicalmultiform}). \begin{equation} \label{eq:formadesambiguada} \begin{array}{rcl} \mbox{disambiguatedform}&\to&\mbox{lexicalmultiform}\\ \mbox{lexicalmultiform}&\to&\mbox{lexicalform}\;[\;\mbox{lexicalform}\;]^*\;\mbox{lemma-queue?}\\ \mbox{lexicalform}&\to&\mbox{lemma}\;\mbox{finetag}\\ \mbox{lemma-queue}&\to&\mbox{lemma}\\ \mbox{finetag}&\to&\mbox{morphsymbol}\;[\;\mbox{morphsymbol}\;]^* \\ \end{array} \end{equation}

Finally, we need also the dictionary of the involved language to train the tagger. This dictionary is used to determine, in combination with the tagset specification, the different ambiguity classes with which the tagger will work.

Figure \ref{fig:dependencias} shows the dependency diagram for the training and the use of the tagger.

\nota{Aquest esquema canviarà amb el nou tagger - Sergio}

\begin{figure} \begin{center} \includegraphics[width=15cm]{diagram} \end{center} \caption{Dependency diagram for the part-of-speech tagger.} \label{fig:dependencias} \end{figure}


\section[Transfer pre-processing]{Auxiliary module: transfer pre-processing module} \label{se:pretransfer} \subsection{Justification} The transfer pre-processing module \texttt{pretransfer} is in charge of separating compound multiwords (see page~\pageref{ss:multipalabras}) and shifting certain parts of multiwords with inner inflection or \emph{split lemma} forms. This module processes the tagger output and generates an entry suitable for the transfer module. The processing performed by this module is necessary for different reasons:

\begin{itemize} \item So that the transfer module can process these units separately in order to deal with, for example, the movement of clitic pronouns when changing from enclitic to proclitic and vice versa. \item So that the bilingual dictionary only has to store information about the lemmas to be translated. If the particles that make up a multiword are included jointly in the bilingual dictionary, the dictionary would have to store an entry for each of the different combinations. By separating compound multiwords and processing multiwords with inner inflection, we can avoid having entries including inflection variations in the bilingual dictionary. \end{itemize}

\subsection{Behaviour and example}

The program replaces each \texttt{<j/>} in the dictionary, that is, each \texttt{+} in the data stream, by a symbol for word end, a blank and a symbol for word beginning. Moreover, if the form is a multiword with split lemma, the queue is moved to the position between the first word of the multiword and its first grammatical symbol.

The task of generating an output which has the original order accepted by the generator, is left to the rules of the transfer module, which are also responsible for creating the compound multiwords which may be required in the target language. In general, the generator works with the same multiwords as the morphological analyser, and with the elements in the same order; that is the reason why this task has to be done in the transfer module.

We show below the result of applying this process to the compound multiword \textit{darlo} ("give it" in Spanish):

\begin{small} \begin{alltt} \$ pretransfer ^dar<vblex><inf>+lo<prn><enc><p3><m><sg>\$ \(\longleftarrow\) \textrm{input} ^dar<vblex><inf>\$ ^lo<prn><enc><p3><m><sg>\$ \(\longleftarrow\) \textrm{output} \end{alltt} \end{small}

As can be seen, it consists only in dividing the lexical forms of a compound multiword into individual lexical forms.

When the input is a multiword with split lemma, the process is as shown in the following example for the Spanish multiword \textit{echarte de menos} ("to miss you"):

\begin{small} \begin{alltt} \$ pretransfer ^echar<vblex><inf>+te<prn><enc><p2><m><sg># de menos\$ ^echar# de menos<vblex><inf>\$ ^te<prn><enc><p2><m><sg>\$ \end{alltt} \end{small}

Here, besides dividing into lexical forms, the module moves the invariable lemma queue into the mentioned position. As you can see, semantic units are maintained after the movement of the invariable queue, since we can consider \textit{echar de menos} a verbal unit with own meaning.

\section{Lexical selection module} \label{se:seleccio_lex}


When the Apertium system is used to translate between less related languages than the ones dealt with in the first stages of the engine, the question of lexical selection becomes significant, because there are more cases, and more critical, in which a source language word can have more than one different translation in the target language. For this reason we created a new module, the lexical selection module, which deals with this problem.

Before going into its characteristics, we will see how the problems of \emph{multiple equivalence} (the fact of existing more than one possible translation in target language for a source language lexical form) are tackled in Apertium in two ways.

On the one hand, we have the situation where there is no big difference in meaning between the multiple equivalents in the target language, and the fact of choosing one or the other can not lead to any translation error. We could say that between these equivalents there is a synonymy or quasi-synonymy relation. In such a case, the linguist chooses one of the lemmas as a translation (generally the most frequent or usual), and adds a direction restriction to the other lemmas (with the attributes \texttt{LR} or \texttt{RL}) so that they are translated in the opposite direction but not in the direction where there are multiple equivalents.

On the other hand, we have the case where there is a clear difference in meaning between the multiple equivalents, which can lead to translation errors if the inappropriate lemma is chosen. These are the cases dealt with the new lexical selection module. The linguist has to encode entries with the attributes \texttt{slr} or \texttt{srl} described in the next section, thus identifying the different translation options; then, the lexical selection module, by means of statistical methods, chooses the translation which is most suitable in a given context.

Sometimes it is not easy to decide whether a multiple equivalence situation should be solved in one way or the other. For example, if there is difference in the meaning of two or more lemmas in the target language, but we think that the lexical selection module will not be capable of choosing the right translation by means of the context, we will follow the first method: choose a fixed translation (the most general, the most suitable in the maximum number of situations) and add a direction restriction to the rest of translations. In the other cases, we will encode the entries so that the decision is left to the lexical selection module.

When we use an Apertium system without lexical selection module, the only way to add entries with different possible translations is the first one, that is, choosing an only translation and marking the other equivalences with a direction restriction. In the event that we use bilingual dictionaries with multiple translations, encoded with the attributes \texttt{slr} or \texttt{srl}, in a system that does not have any lexical selection module, a style sheet will convert these entries designed for a lexical selection module into entries with direction restrictions \texttt{LR} or \texttt{RL}, so that one of the multiple equivalents (the one chosen as default entry by the linguist) becomes the fixed translation of the source language lemma.

As examples of bilingual equivalencies that should have a direction restriction, we can give the translation pairs \texttt{ca-es} \emph{encara – aún/todavía} ("still") and \emph{sobtat – súbito/repentino} ("sudden"), the first one of which could be encoded like this: \begin{alltt} \begin{small}

<e r="LR"> <p> <l>aún<s n="adv"/></l> <r>encara<s n="adv"/></r> </p> </e> <e> <p> <l>todavía<s n="adv"/></l> <r>encara<s n="adv"/></r> </p> </e> \end{small} \end{alltt}

As examples of the second case (multiple equivalents with big difference in meaning) we have the pairs \texttt{es-ca} \emph{hoja – full/fulla} ("sheet/leaf") and \emph{muñeca – nina/canell} ("doll/wrist"), as well as the \texttt{en-ca} examples shown in page \pageref{entrades_lextor}, where it is described how to specify these multiple equivalents in the bilingual dictionary.

\begin{figure} {\footnotesize \setlength{\tabcolsep}{0.5mm} \begin{center} \begin{tabular}{ccccccccc} \\ \parbox{0.95cm}{source language text} \\ $\downarrow$ \\ \framebox{\parbox{1.0cm}{de-for\-matter}} $\rightarrow$ & \framebox{\parbox{0.6cm}{morph. anal.}} $\rightarrow$ & \framebox{\parbox{1.0cm}{POS tagger}} $\rightarrow$ & \framebox{\parbox{0.6cm}{lex. select.}} $\rightarrow$ & \framebox{\parbox{0.85cm}{struct. transf.}} $\rightarrow$ & \framebox{\parbox{0.6cm}{morph. gen.}} $\rightarrow$ & \framebox{\parbox{1.2cm}{post\-generator}} $\rightarrow$ & \framebox{\parbox{1.0cm}{re-for\-matter}} \\ & & & & $\updownarrow$ & & & $\downarrow$ \\ & & & & \framebox{\parbox{0.8cm}{lex. transf.}} & & & \parbox{0.95cm}{target language text} \\ \end{tabular} \end{center} } \caption{The nine modules that build the assembly line in the version 2 of the machine translation system Apertium.} \label{fig:moduls} \end{figure}

Figure~\ref{fig:moduls} shows the new assembly line of the version 2 of Apertium.\footnote{This figure substitutes the figure \ref{fg:modules} in page \pageref{pg:modules} which represents the version 1 of Apertium.} \nota{MG: caldria canviar la figura de la pàgina 6 per aquesta d’aquí?} The module in charge of the lexical selection (lexical selector) runs after the part-of-speech tagger and before the structural transfer module; therefore, this new module works only with source language information.

Section~\ref{se:preprocessament} next describes the pre-processing that must be done on a bilingual dictionary containing more than one translation per entry (whether the system uses a lexical selector or not), and Section~\ref{se:lextor} describes how the lexical selector works and how it has to be trained.

\subsection{Pre-processing of the bilingual dictionaries }\label{se:preprocessament}

Bilingual dictionaries have been modified to allow the specification of more than one translation per entry (refer to Section \ref{dic_lextor} to learn how to write such dictionary entries); this fact makes it necessary to pre-process these dictionaries, since the Apertium engine works with compiled dictionaries in which there is only one possible translation for each word.

The pre-processing of dictionaries is done automatically during compilation, therefore the final user does not need to perform any specific action.

\subsubsection{Pre-processing without lexical selection module}

When bilingual dictionaries with multiple equivalents are used in a system where there is no lexical selection module, the pre-processing is done by the application of the style sheet \texttt{translate-to\–de\-fault\–e\-qui\-va\-lent.xsl}. This style sheet turns dictionaries with multiple translations per entry into dictionaries with only one translation per entry; to do this, it chooses as translation the entry marked as default, and adds a direction restriction (\texttt{LR} or \texttt{RL} as applicable) to the other entries, so that they are only translated in the translation direction where there is no equivalent multiplicity. The style sheet is called from the \texttt{Makefile}.

To put an example, the result of applying the style sheet on the first three entries shown in page \pageref{entrades_lextor} is the following:

\begin{alltt} \begin{small} <e> <p> <l>flat<s n="n"/></l> <r>pis<s n="n"/><s n="m"/></r> </p> </e>

<e r="LR"> <p> <l>floor<s n="n"/></l> <r>pis<s n="n"/><s n="m"/></r> </p> </e>

<e r="RL"> <p> <l>floor<s n="n"/></l> <r>terra<s n="n"/><s n="m"/></r> </p> </e> \end{small} \end{alltt}

\subsubsection{Preprocessing with lexical selection module}

If the Apertium system works with a lexical selection module, the bilingual dictionary must be pre-processed in order to obtain: \begin{itemize} \item a monolingual dictionary that, for each source language word (for example \emph{look}) delivers all the possible translation marks or equivalents (\texttt{look\_\_mirar D} and \texttt{look\_\_semblar}); this dictionary will be used by the lexical selection module; and

\item a new bilingual dictionary that, given a word with the lexical selection already done (for example \texttt{look\_\_semblar}) delivers the translation (\emph{semblar}); this will be the bilingual dictionary to be used in the lexical transfer.


This pre-processing is automatically done by means of the following software during dictionary compilation: \begin{itemize} \item \texttt{apertium-gen-lextormono}, that receives three parameters: \begin{itemize} \item the translation direction for which you want to generate the monolingual dictionary used in the lexical selection; \texttt{lr} for the translation left to right, and \texttt{rl} for the translation right to left; \item the monolingual dictionary to be pre-processed; and \item the file where the output monolingual dictionary has to be written. \end{itemize}

\item \texttt{apertium-gen-lextorbil}, that receives three parameters: \begin{itemize} \item the translation direction (\texttt{lr} or \texttt{rl}) for which you want to generate the bilingual dictionary to be used by the lexical transfer module; \item the bilingual dictionary to be pre-processed; and \item the file where the output bilingual dictionary has to be written. \end{itemize} \end{itemize}

\subsection{Execution of the lexical selection module}\label{se:lextor}

The module responsible for the lexical selection runs after the part-of-speech tagger and before the structural transfer (see Figure~\ref{fig:moduls} in page~\pageref{fig:moduls}); therefore, it uses only information from the source language. However, during the training of the module, target language information is also used.


To train the lexical selection module, a corpus in the source language and another one in the target language are required; they do not need to be related. Both corpora must be pre-processed before the training. This pre-processing, consisting in analysing the corpora and performing the POS disambiguation, can be done with \texttt{apertium-prepro\-cess\–cor\-pus\–lex\-tor}.

The training of the module that performs the lexical selection consists of the following tasks:\footnote{The training of the models used for the lexical selection has been automated in all the packages using it. Furthermore, all the software mentioned has its UNIX manual page}

\begin{enumerate} \item Obtain the list of words that will be ignored when performing lexical selection (\emph{stopwords}). This list can be done manually or using \texttt{apertium-gen-stopwords-lextor}; \item Obtain the list of (source language) words that have more than one translation in the target language, using \texttt{apertium-gen-wlist-lextor}; \item Translate to the target language all the words obtained in the previous step, using \texttt{apertium-gen-wlist-lextor-translation}; \item Running \texttt{apertium-lextor –trainwrd} and using the target language pre-processed corpus, train a word co-occurrence model for the words obtained in the previous step; \item Running \texttt{apertium-lextor –trainlch} and using the source language pre-processed corpus, the dictionaries generated by the programs mentioned in Section~\ref{se:preprocessament} and the word co-occurrence models calculated in the previous step, train a co-occurrence model for each of the translation marks of those words that can have more than one translation in the target language. \end{enumerate}


The word co-occurrence models calculated for each translation mark as described in the previous section provide the information required to perform lexical selection with information from the context.

Lexical selection is done by \texttt{apertium-lextor –lextor}; the formats used to communicate with the rest of the modules of the translation engine are:

\begin{description} \item [Input:] text in the same format as the input for the structural transfer module, that is, text analysed and disambiguated, with invariable queues of multiwords moved before morphological tags. \item [Output:] text in the same format, but with the translation mark to be used when executing lexical transfer. \end{description}

The following example illustrates the input/output formats used by the lexical selector (we have assumed in the example that only the English verb \emph{get} has more than one translation equivalent in the dictionaries): \begin{itemize} \item Source language text (English): \emph{To get to the city centre} \item Lexical selector input: \verb!^To<pr>$! \verb!^get<vblex><inf>$! \verb!^to<pr>$! \verb!^the<det><def><sp>$! \verb!^city<n><sg>$! \verb!^centre<n><sg>$! \item Translation marks in the en-ca bilingual dictionary for the verb \emph{get}: \texttt{rebre}, \texttt{agafar}, \texttt{arribar}, \texttt{aconseguir D} \item Lexical selector output: \verb!^To<pr>$! \verb!^get__arribar<vblex><inf>$! \verb!^to<pr>$! \verb!^the<det><def><sp>$! \verb!^city<n><sg>$! \verb!^centre<n><sg>$! \end{itemize}

\newpage \section{Structural transfer module} \label{ss:transfer}

\nota{Faena per fer (mlf): \begin{itemize} \item Hi ha bastants vacil·lacions en la terminologia usada per a referir-se a conceptes i en els noms usats per als programes. \item He intentat substituir en cada cas l’expressió \emph{per defecte} per una altra més adequada; però caldrà distingir en quin cas ens trobem en cada cas. \end{itemize}}


In 2007, Apertium incorporated a more advanced structural transfer system than the one used until then; it became necessary when we started developing machine translators for less related language pairs in comparison with the ones dealt with before, such as the \emph{English}–\emph{Catalan} translator.

This enhanced transfer system is made of three modules, the first one of which can be used in isolation in order to run a \textbf{shallow-transfer} system (which is the transfer system used so far for related language pairs such as \emph{Spanish}–\emph{Catalan} or \emph{Spanish}–\emph{Galician}). When the system is used for less related language pairs and, therefore, an \textbf{advanced transfer} becomes necessary, the three transfer modules will be executed.

The two transfer systems differ in the number of passes over the input text. The shallow-transfer system makes structural transformations with a single pass of the rules, which detect sequences or \emph{patterns} of lexical forms and perform on them the required verifications and changes. On the other hand, the advanced transfer system works with a new architecture that allows to detect \emph{patterns of patterns} of lexical forms with three passes, done by its three modules.

We describe next the characteristics of the structural transfer system. Section \ref{functransfer} describes the shallow-transfer system and Section \ref{apertium2}, the advanced transfer system. The description of the shallow-transfer system is also applicable to the first module of the advanced transfer system, with the differences mentioned in that section. Section \ref{formatotransfer} describes the format used to create rules in both systems. In Section \ref{noutransfer} there is a detailed description of how the three modules of the advanced transfer system work, and finally, Section \ref{ss:preproceso_transfer} describes the pre-processing required by the modules.

\subsection{Shallow-transfer} \label{functransfer}

In this system, only the first of the three modules that compose the advanced transfer system is used. This module is called \emph{chunker}.

The design of the language and the compiler used to generate the structural transfer module is largely based upon the MorphTrans language described in \cite{garridoalenda01p} and used by the MT systems \textsf{interNOSTRUM} \cite{canals01b,garridoalenda01p,garrido99j} (Spanish–Catalan) and \textsf{Traductor Universia} \cite{garrido03p, gilabert03j} (Spanish–Portuguese), developed by the Transducens group at the Universitat d’Alacant.

The transfer process is organized around patterns representing fixed-length sequences of source language lexical forms (SLLFs) (see page~\pageref{pg:FSFL} for a description of lexical form (LF)); a sequence follows a certain pattern if it contains the sequence of lexical forms of the pattern. Patterns do not need to be constituents or phrases in the syntactic sense: they are mere concatenations of lexical forms that may need a conjoint processing additional to the simple word-for-word translation, due to the grammatical divergences between SL and TL (gender and number changes, reorderings, prepositional changes, etc). The catalogue of patterns defined for a certain language is selected with a view to covering the most common structural transformations. When source language and target language are syntactically similar, as is the case between Spanish, Catalan and Galician, simple rules based on sequences of lexical categories achieve a reasonable translation quality.

The transfer module detects, in the SL, sequences of lexical forms that match one of the patterns previously defined in the pattern catalogue, and processes them applying the corresponding structural transfer rule, doing at the same time the lexical transfer by reading the bilingual dictionary.

The \emph{pattern detection} phase occurs as follows: if the transfer module starts to process the $i$-th SLLF of the text, $l_i$, it tries to match the sequence of SLLFs $l_i, l_{i+1}, \ldots$ with all of the patterns in its pattern catalogue: the longest matching pattern is chosen, the matching sequence is processed (see below), and processing continues at SLLF $l_{i+k}$, where $k$ is the length of the pattern just processed. If no pattern matches the sequence starting at SLLF $l_i$, it is translated as an isolated word an processing restarts at SLLF $l_{i+1}$ (when no patterns are applicable, the system resorts to word-for-word translation). Note that each SLLF is processed only once: patterns do not overlap; hence, processing occurs left to right and in distinct "chunks".

In the \emph{pattern processing } phase, the system takes the detected sequence of SLLFs and builds (using a program to consult the bilingual dictionary) a sequence of TL lexical forms (TLLFs) obtained after the application of the operations described in the rule associated to the detected pattern (reordering, addition, replacement or deleting of words, inflection changes, etc.). The information that does not change is automatically copied from SL to TL. The resulting data, that is, the lemmas with their associated morphological tags, are sent to the generator, which creates the inflected forms.

For instance, the Spanish sequence \emph{una señal inequívoca} ("an unmistakable signal"), that would go from the tagger to the transfer module in the following format~\footnote{The example has been presented in a way that it does not contain superblanks with format information, so that the linguistic side of the transformation is clearer. See Chapter \ref{se:flujodatos}.}:\\

\begin{alltt} \begin{small} \textasciicircum\textbf{uno}<det><ind><f><sg>\$ \textasciicircum\textbf{señal}<n><f><sg>\$ \textasciicircum\textbf{inequívoco}<adj><f><sg>\$ \end{small} \end{alltt}

\noindent{would be detected as a pattern by a rule for determiner–noun–adjective.} The transfer module would consult the bilingual dictionary to get the Catalan equivalents and, as it would detect a gender change in the word \emph{señal} (its Catalan translation \emph{senyal} is masculine), it would propagate this change to the determiner and the adjective to deliver the output sequence:\\

\begin{alltt} \begin{small} \textasciicircum\textbf{un}<det><ind><m><sg>\$ \textasciicircum\textbf{senyal}<n><m><sg>\$ \textasciicircum\textbf{inequívoc}<adj><m><sg>\$ \end{small} \end{alltt}

\noindent{which the generation module would turn into the Catalan inflected sequence: \emph{un senyal inequívoc}.}

The task of most rules is to ensure gender and number agreement in simple noun phrases (determi\-ner–noun, determiner–noun–adjective, determiner–adjective–noun, determiner–adjective, etc.), provided that there is agreement between the SLLFs of the detected pattern. These rules are required either because the noun changes its gender or number between SL and TL (as in the previous example) or because gender or number in the TL have to be determined due to the fact that it was ambiguous in SL for some of the words (for example, the Catalan determiner \emph{cap} can be translated into Spanish as \emph{ningún} (masc.) or \emph{ninguna} (fem.) depending on the accompanying noun: \emph{cap cotxe} (\texttt{ca}) $\rightarrow$ \emph{ningún coche} (\texttt{es}) and \emph{cap casa} (\texttt{ca}) $\rightarrow$ \emph{ninguna casa} (\texttt{es})). Furthermore, there other rules defined to solve frequent transfer problems between Spanish, Catalan and Galician, such as, among others:


\item rules to change prepositions in certain constructions: \emph{in Barcelona} (\texttt{es}) $\rightarrow$ \emph{a Barcelona} (\texttt{ca}); \emph{consiste en hacer} (\texttt{es}) $\rightarrow$ \emph{consisteix a fer} (\texttt{ca});

\item rules to add/remove the preposition \emph{a} in certain Galician modal constructions with the verbs \emph{ir} and \emph{vir}: \emph{vai comprar} (\texttt{gl}) $\rightarrow$ \emph{va a comprar} (\texttt{es});

\item rules for articles before proper nouns: \emph{ve la Marta} (\texttt{ca}) $\rightarrow$ \emph{viene Marta} (\texttt{es});

\item lexical rules, for instance, to decide the correct translation of the adverb \emph{molt} (\texttt{ca}) into Spanish (\emph{muy, mucho}) or of the adjective \emph{primeiro} (\texttt{gl}) or \emph{primer} (\texttt{ca}) into Spanish (\emph{primer, primero});

\item rules to displace atonic or clitic pronouns, whose position in Galician is different to that in Spanish (proclitic in Galician and enclitic in Spanish or vice versa): \emph{envioume} (\texttt{gl}) $\rightarrow$ \emph{me envió} (\texttt{es}); \emph{para nos dicir} (\texttt{gl}) $\rightarrow$ \emph{para decirnos} (\texttt{es}).


\emph{Multiwords} (its different types are described in page~\pageref{ss:multipalabras}) are processed in a special way in this module:

\begin{itemize} \item \emph{Multiwords without inflection}, made of only one lexical form, do not need any special processing, since they are treated like other LFs. \item In the case of \emph{compound multiwords}, that is, multiwords formed by more than one \emph{lexical form}, each one with its own grammatical symbols and joined to each other with the element \texttt{<j>} in the dictionary entry (which corresponds to the symbol ’+’ in the data stream), the auxiliary module \texttt{pretransfer} (see \ref{se:pretransfer}), located before this module, separates the different lexical forms so that they reach the transfer module as independent LFs. If we want to join them again so that they reach the generator as multiwords (as is the case of enclitic pronouns in our system), it has to be done by means of a transfer rule, using the \texttt{<\textbf{mlu}>} element (described later, in section \ref{ss:mlu}). In page~\pageref{regla_verbo2} you can find an example of a rule for joining enclitic pronouns to the verb. \item As for \emph{multiwords with inner inflection}, the \texttt{pre\-trans\-fer} module moves the lemma queue (the invariable part) to place it after the lemma head (the inflective form), thus making possible to find the multiword in the bilingual dictionary. This kind of multiwords must be processed by a structural transfer rule which replaces the lemma queue in its proper position. This is done by using, in the output of the rule, the attributes \texttt{lemh} “lemma head” and \texttt{lemq} “lemma queue”) of the \texttt{<\textbf{clip}>} element. See page~\pageref{ss:lu} for a more detailed description of the use of this element, and page \pageref{regla_verbo1} to see two rules where these attributes are used. \end{itemize}

\subsection{Advanced transfer} \label{apertium2}

The shallow-transfer architecture described in the previous section is based, as we have seen, in the automatic handling of word co-occurrence patterns by means of rules defined by the user. This model considers two levels from the point of view of the nature of data: a basic level we call \textit{lexical level}, which handles words and the tasks of consulting and changing its characteristics (lemma and tags), besides translating individual lemmas by asking the bilingual dictionary; and another level we call \textit{word pattern level}, which is in charge of doing, when applicable, reorderings of the words that build these patterns, as well as changes in the properties of words that depend on the specific pattern that has been detected. All this process of detection and manipulation of words and patterns is carried out in a single pass.

In contrast, the new advanced transfer architecture is defined as a transfer system in three levels and three passes. The first two levels, lexical and pattern level, are the same ones of the shallow-transfer system. The new added level is a level of \emph{patterns of patterns} of words. The aim of this new processing level is to allow the handling and interaction of patterns of words in a similar way as words are handled in the patterns of the shallow system. With this new structure we intend to achieve a more appropriate handling of all transformations that may be required when translating from one language to another. We want to emphasize that the definition of word patterns in the shallow-transfer system does not need to be the same as the definition of word patterns in the advanced system: we pretend that, in the latter, patterns have a \textit{spirit} of phrases that does not exist in the previous system. Therefore we will use the term \textit{chunk} to refer to word sequences in the advanced transfer system.

The advanced transfer system is organized in three passes. According to the Apertium processing mode, these three passes are carried out by three different modules (programs):

\begin{itemize} \item \texttt{chunker}: identifies chunks, translates word for word, and carries out required reorderings and morphosyntactic data propagation inside the chunk (for example, to maintain agreement). Besides, it creates the chunks that will be processed by the next module. The \texttt{chunker} has the option of running as a single module in a shallow-transfer system. This is controlled by an attribute in the \texttt{<transfer>} element.

\item \texttt{interchunk}: this module receives the chunks generated by the \texttt{chunker} and is able to reorder them, modify the “syntactic information” associated to each chunk and, finally, output the chunks in the new order and with the new properties, creating new chunks if needed. \item \texttt{postchunk}: it receives the chunks modified by the interchunk and carries out final tasks concerning modification of the words contained in each chunk and printing of the text contained in chunks in the format accepted by the generator. \end{itemize}

In the following lines we specify the format of the chunks that circulate between the modules of the transfer system (Section \ref{sec:format}) and the letter case handling in chunks (Section \ref{ss:majuscules}), which is different from case handling of individual lexical forms in a shallow-transfer system.

The following section, \ref{formatotransfer}, describes the format of transfer rules, which is the same for the three modules and the two transfer modes, with little differences. Finally, after this description, in \ref{noutransfer} you will find a more detailed explanation of the three modules that make up an advanced transfer system.

\subsubsection{Chunk format} \label{sec:format}

Communication between \texttt{chunker} and \texttt{interchunk}, as well as between \texttt{interchunk} and \texttt{postchunk}, is performed through sequences of chunks. We define $C$ as a \emph{sequence of chunks}, that has the form: $$ C=b_{0}c_{1}b_{1}c_{2}b_{2} \ldots b_{k-1}c_{k}b_{k} $$

where each $b_i$ is a \textit{superblank}, and each $c$ is a \emph{chunk}. A chunk $c$ is defined as a string \verb!^!$F$\verb!{!$W$\verb!}$! that contains the following information:

\begin{itemize} \item $F$ is the \emph{lexical pseudoform}\nota{help: pseudoforma lèxica = lexical pseudoform or pseudolexical form}; it is a string that has the form $fE$, where $f$ is the \textit{pseudolemma} of the chunk, and $E=e_{1}e_{2} \ldots$ is a sequence of grammatical symbols called \emph{chunk symbols}. Changing these symbols will cause the changing of the morphological information of words in the chunk, if this information is linked to these parameters. \item $W=b_{0}w_{1}b_{1}w_{2}b_{2} \ldots w_{k}b_{k}$ is the sequence of words $w_i$ sent by the chunker with the intermediate \textit{superblanks} $b_i$. These words have the same format in both transfer systems, that is, an individual word $w_i=$\verb!^!$l_{i}E_{i}$\verb!$! contains lemma $l_i$ and grammatical symbols $E_i$, some of which can be \emph{references or links to the symbols} of the chunk and are identified with natural numbers \texttt{<1>}, \texttt{<2>}, \texttt{<3>}, etc. These references to symbols correspond, in the specified order, to the symbols of $E$. \end{itemize}

The following is a use example of the described format, with the text \emph{el gat} ("the cat"):

\begin{small} \begin{alltt} \verb!^!det_nom<SN><m><pl>\verb!{^!el<det><def><2><3>$[ <a href="http://www.ua.es">]^gat<n><2><3>$\verb!}$![</a>] \end{alltt} \end{small}

The characters \verb!{! and \verb!}!, if present in the original text, must be escaped with a backslash \verb!\!.

\subsubsection{Letter case handling} \label{ss:majuscules}

For each chunk, the case of words is determined by the case of the pseudolemma of the chunk, taking into account the following rules:


\item When all the letters of the pseudolemma are in lower case: the case state of words is not modified. \item When the first letter of the pseudolemma is in upper case and the rest are in lower case: in the module \texttt{postchunk}, when words are printed, the letter that is the first of the chunk after all the possible word reorderings will be put in upper case \nota{MG: and the rest will be put in lower case except proper nouns? is this correct?}. \item When all the letters of the pseudolemma are in upper case: all the words will remain upper case. \end{itemize}

It is required that the words in the chunk are not capitalized unless they are proper nouns, so as to avoid the postchunk module having to look for the word that has to lose capitalization, if this is the case\nota{MG: I am not sure I understand this}. This task belongs to the \texttt{chunker} module and is done with a macro or similar mechanism.

%\settocdepth{subsection} \subsection{Format specification for structural transfer rules} \label{formatotransfer}

This section describes the format in which structural transfer rules are written. In the Appendix, in sections~\ref{ss:dtdtransfer}, \ref{ss:dtdinterchunk} and \ref{ss:dtdpostchunk}, there is the formal definition (DTD).

Structural transfer rules files have two well-differentiated parts: one for the declaration of the elements to be used in rules, and another one for the rules themselves.\\

In the \textbf{declaration} part we find:


\item A series of declarations of \emph{lexical categories}, which specify those lexical forms that will be treated as a particular category and will be detected by patterns. The linguist may include any data about the lexical form to define a category; categories can be very generic (i.e. all the nouns) or very specific (i.e. only those determiners that are demonstrative feminine plural). \item A series of declarations of the \emph{attributes} we want to detect in lexical forms (like \emph{gender}, \emph{number}, \emph{person} or \emph{tense}), to perform with them the required transformation operations and send the resulting data in the output of the rules. The declaration of an attribute contains the name of the attribute and the possible values it can take in a lexical form (in general they correspond to the morphological attributes that characterize the form): for example, the attribute \emph{number} can take the values \emph{singular}, \emph{plural}, \emph{singular-plural} (for invariable lexical forms, like \emph{crisis} in Spanish) and \emph{number to be determined} (for TL lexical forms with different forms for \emph{singular}–\emph{plural}, but whose number can not be determined in the translation due to the fact that the SL lexical form is invariable in number, see explanation in page \pageref{pg:GD}). If inside the rule, outside of the pattern, one wishes to refer to any of the lexical categories defined in the previous point (to perform tests or actions on them), it will be also necessary to define attributes for them.

\item A series of declarations of \emph{global variables}, which are used to transfer values of active attributes inside a rule, or from one rule to the ones applied subsequently.

\item A section for the \textit{definition of string lists}, generally lists of lemmas, which will be used to make searches on them for a certain value to perform a specific transformation.

\item A series of declarations of \emph{macro-instructions}; macro-instructions contain sequences of frequently used instructions, and can be included in different rules (for example, a macro-instruction to ensure gender and number agreement between two lexical forms of a pattern).


In the \textbf{structural transfer rules} we find:

\begin{itemize} \item The definition of the pattern that will be detected, specified as a sequence of lexical categories as they have been defined in the declaration part. It must be noted that, if a sequence of lexical forms matches two different rules, firstly, the longest is chosen, and secondly, for rules of the same length, the one defined before is chosen.

\item The process part of the rules, where actions to be performed on SLLF are specified, and the TL pattern is built.

\end{itemize} \nota{Assegurem-nos que totes les sigles estan definides}

In the following pages we describe in detail the characteristics of all the elements used in rules.

\subsubsection{Element \texttt{<transfer>}}

(\textit{Only in the chunker module})

This is the root element of the \texttt{chunker} module and contains all the rest of the elements of the structural transfer rules file of this module.

Its attribute \texttt{default} can take two values: \begin{itemize}

\item \texttt{lu}: it means that it will run in shallow mode, that is, as only transfer module in a shallow-transfer system and, therefore, no special action will be done on words not detected by any pattern

\item \texttt{chunk}: it means that it will run in advanced mode and, therefore, when a word is not recognized by any rule, a chunk will be created to encapsulate it, so that it can be processed by the next transfer modules of an advanced transfer system.


The default value is \texttt{lu}.

\subsubsection{Element \texttt{<interchunk>}}

(\textit{Only in interchunk})

This is the root element of the \texttt{interchunk} module and contains all the rest of the elements of the structural transfer rules file of this module.

\subsubsection{Element \texttt{<postchunk>}}

(\textit{Only in postchunk})

This is the root element of the \texttt{postchunk} module and contains all the rest of the elements of the structural transfer rules file of this module.

\subsubsection{Element for category definition section \\\texttt{<section-def-cats>}} \nota{Atenció a l’ús polisèmic del mot \emph{categoria} en el document}

This section contains the definition of the lexical categories that will be used to create the patterns used in rules. Each definition is made with a \texttt{<\textbf{def-cat}>}.

\subsubsection{Element for category definition \texttt{<def-cat>}}

Each category definition has a mandatory name \texttt{n} (e.g. \texttt{det}, \texttt{adv}, \texttt{prep}, etc.) and a list of categories (\texttt{<\textbf{cat-item}>}) that define it. The name of the category can not contain accents.

\subsubsection{Element for category \texttt{<cat-item>}}

This element has two well-differentiated uses depending on the module it is used in.

\paragraph{Use in chunker (shallow transfer and advanced transfer)}

This element defines the lexical categories that will be used in patterns, that is, that the linguist wishes to detect in the source text. These categories are defined by a subsequence of the fine tags (see definition in page~\pageref{ss:introtagger}) that deliver both the morphological analyser and the tagger\footnote{Please note that throughout the different linguistic modules, different lexical categorizations are used: in morphological dictionaries, lemmas are accompanied by a fine tag (for instance, \texttt{\emph{<n><m><pl>}} for plural masculine nouns); the POS tagger groups these fine tags in more general tags (for instance, the category \texttt{NOUN} for all the nouns), although its output is again the whole fine tag of each LF; finally, in the transfer module, the fine tags of LFs are grouped again in more general categories (although it is also possible to define particularized categories) depending on the type of lexical forms that one wants to detect in patterns.}.

Each \texttt{<\textbf{cat-item}>} element has a mandatory attribute \texttt{tags} whose value is a sequence of grammatical symbols separated by a dot; this sequence is a subsequence of the fine tag, that is, of the sequence of grammatical symbols that defines every possible lexical form delivered by the tagger. According to this, a category represents a certain set of lexical forms. We must define as many different categories as kinds of lexical forms we want to detect in patterns. Thus, if we want to detect all the nouns to perform certain actions on them, we will create a category defined with the grammatical symbol \texttt{n}. On the other hand, if we want to detect all the plural feminine nouns, we will have to define a category using the symbols \texttt{n} \texttt{f} and \texttt{pl}.

When, for the set of lemmas we want to include in a category, a grammatical symbol used to define the category is followed by other grammatical symbols, the character \texttt{"*"} is used. For example, \texttt{tags}=\texttt{"n.*"} covers all the lexical forms that contain this symbol, such as the Spanish nouns \texttt{casa<n><f><pl>} or \texttt{coche<n><m><sg>}. On the other hand, when after the used symbol there can not be any other symbol, the asterisk is not included: for example, \texttt{tags}=\texttt{"}\texttt{adv"} will cover all adverbs, since in our system they are characterized with only one grammatical symbol. The asterisk can also be used to signal the existence of preceding symbols: \texttt{tags}=\texttt{"*.f.*"} includes all feminine lexical forms, whichever category they are. Furthermore, an optional attribute, \texttt{lemma}, can be used to define lexical forms on the basis of its lemma (see Figure \ref{fig:cat-item}).

\begin{figure} \begin{small} \begin{alltt} <\textbf{def-cat} \textsl{n}="nom"/> <\textbf{cat-item} \textsl{tags}="n.*"/> </\textbf{def-cat}>

<\textbf{def-cat} \textsl{n}="que"/> <\textbf{cat-item} \textsl{lemma}="que" \textsl{tags}="cnjsub"/> <\textbf{cat-item} \textsl{lemma}="que" \textsl{tags}="rel.an.mf.sp"/> </\textbf{def-cat}> \end{alltt} \end{small} \caption{Use of the \texttt{<\textbf{cat-item}>} element to define two categories, one for nouns without lemma specification (\emph{nom}), which includes all lexical forms whose first grammatical symbol is \emph{n}, and another one with associated lemma (\emph{que}), which has two subsequences of fine tags, to include the \emph{que} conjunction and the \emph{que} relative pronoun.} \label{fig:cat-item} \end{figure}

\paragraph{Use in interchunk}

It is used like in the \texttt{chunker} module, but here, instead of being defined with the grammatical symbols of lexical forms, it is defined with the symbols of the chunks delivered by the \texttt{chunker}. For example, in the case that we want to define a category to detect all the determined noun phrases, we will define it with the symbols \texttt{NP} and \texttt{DET} if this is how we tagged these chunks by means of the \texttt{<tag>} instructions contained in the \texttt{<chunk>} element (see \ref{ss:chunker}). You can also use the optional attribute \texttt{lemma} to refer to the \emph{pseudolemma} of the chunk. So, its formal characteristics are the same in the modules \texttt{chunker} and \texttt{interchunk}, with the difference that in the former they are used to detect lexical forms, and in the latter, to detect chunks.

\paragraph{Use in postchunk}

In this module, this element only has the mandatory attribute \texttt{name}, which refers to the name of the chunk,

\nota{MG: abans deia ’al nom de la regla’, comentari mlf: De la regla o del patró?} without tags, since in the \texttt{postchunk} module only the pseudolemma (name of the chunk) is used for detection. Case is ignored in detection, because the pseudolemma is used to convey information about the case of the chunk. (See Figure \ref{fig:cat-item-postchunk}).

\begin{figure} \begin{small} \begin{alltt} <\textbf{def-cat} \textsl{n}="det-nom"/> <\textbf{cat-item} \textsl{name}="det-nom"/> </\textbf{def-cat}> \end{alltt} \end{small} \caption{Use of the \texttt{<\textbf{cat-item}>} element in the postchunk to detect chunks of determiner-noun.} \label{fig:cat-item-postchunk} \end{figure}

\subsubsection{Element for category attribute definition section \\\texttt{<section-def-attrs>}}

This section is to describe the attributes that will be extracted from the categories detected by the pattern and that will be used in the action part of the rules. Each attribute is defined by a \texttt{<\textbf{def-attr}>} tag.

\nota{De vegades les etiquetes aprareixen en el text en negretes i de vegades sense negretes. Decidim-nos per una tipografia i usem-la en tot el document.}

\subsubsection{Element for category attribute definition \\\texttt{<def-attr>}}

Each \texttt{<\textbf{def-attr}>} defines an attribute regarding morphological information (both inflection information –gender, number, person, etc.–, and categorial –verb, adjective, etc–) by specifying a list of category attribute (\texttt{<\textbf{attr-item}>}) elements, and has a mandatory unique name \texttt{n}. Therefore, an attribute is defined on the basis of the grammatical symbols that can be found in a given lexical form. Each attribute extracts, from the lexical forms of the pattern, the symbols that these contain among the set of possible values defined.

\subsubsection{Element for category attribute \texttt{<attr-item>}}

Each category attribute element represents one of the possible values the attribute can take. For example, the attribute for number \texttt{nbr} can take the values singular \texttt{sg}, plural \texttt{pl}, singular–plural \texttt{sp} and number to be determined \texttt{ND}. These values are a subsequence of the morphological tags that characterize each lexical form, and are specified in the \texttt{tags} attribute of the element, separated by a dot if there is more than one. In Figure \ref{fig:attr-item} you can find an example for the attributes for \emph{number} and \emph{noun}. \nota{Potser s’hauria d’explicar per què s’ha triat el nom \emph{a\_nom} en la figura}

Compare the definition of the attribute for number in this figure (with all possible values and without asterisks) with the definition of the category for noun in Figure \ref{fig:cat-item}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{def-attr} \textsl{n}="nbr"/> <\textbf{attr-item} \textsl{tags}="sg"/> <\textbf{attr-item} \textsl{tags}="pl"/> <\textbf{attr-item} \textsl{tags}="sp"/> <\textbf{attr-item} \textsl{tags}="ND"/> </\textbf{def-attr}>

<\textbf{def-attr} \textsl{n}="a_nom"/> <\textbf{attr-item} \textsl{tags}="n"/> <\textbf{attr-item} \textsl{tags}="n.acr"/> </\textbf{def-attr}>

\end{alltt} \end{small} \caption{Definition of the category attribute \texttt{nbr} for \emph{number}, which can take the values \emph{singular}, \emph{plural}, \emph{singular-plural} or \emph{number to be determined}, and the category attribute \texttt{a\_nom} for \emph{noun}, which can take the values of the symbols \emph{n} or \emph{n acr}.} \label{fig:attr-item} \end{figure}

\subsubsection{Element for variable definition section \\\texttt{<section-def-vars>}}

In this section, \texttt{<\textbf{def-var}>} tags are used to define global string variables, that will be used to transfer information inside the rule and from one rule to another one (for example, to transmit information on gender or number between two patterns)

\nota{Que quede clar que aquesta transferència d’una regla a altra es fa només d’una aplicació d’una regla a l’aplicació d’altra regla en un moment posterior, o d’esquerra a dreta}

\subsubsection{Element for variable definition \texttt{<def-var>}} \label{ss:defvar} The definition of a global string variable has a mandatory unique name \texttt{n} that will be used to refer to it inside a rule. Variables contain strings that describe state information, such as the existence of agreement between two elements, the detection of a question mark in SL that should be deleted in TL, etc.

\subsubsection{Element for string lists definition section \\\texttt{<section-def-lists>}} In this section, lists are defined (with \texttt{<\textbf{def-list}>} tags) that will be used to do string searches. These lists can be used to group word lemmas that have a common feature (i.e. verbs expressing movement, adjectives expressing emotions, etc.). This section is optional.

\subsubsection{Element for string lists definition \texttt{<def-list>}} This element is used to name the string list, with the attribute \texttt{n}, and to encapsulate the list defined by one or more \texttt{<\textbf{list-item}>} elements. An example of its use can be found in Figure \ref{fig:deflist}.

\subsubsection{Element for string list item \texttt{<list-item>}} It defines, with the value of the attribute \texttt{v}, the specific string that is included in the definition of the list. An example of its use can be found in Figure \ref{fig:deflist}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{def-list} n="verbos_est"> <\textbf{list-item} v="actuar"/> <\textbf{list-item} v="buscar"/> <\textbf{list-item} v="estudiar"/> <\textbf{list-item} v="existir"/> <\textbf{list-item} v="ingressar"/> <\textbf{list-item} v="introduir"/> <\textbf{list-item} v="penetrar"/> <\textbf{list-item} v="publicar"/> <\textbf{list-item} v="treballar"/> <\textbf{list-item} v="viure"/> <\textbf{/def-list}> \end{alltt} \end{small} \caption{Definition of a list of Catalan lemmas. These lemmas are used in the rule in Figure \ref{fig:in}.} \label{fig:deflist} \end{figure}

\subsubsection{Element for macro-instruction definition section \\\texttt{<section-def-macros>}}

This section is for the definition of macro-instructions that contain pieces of code used frequently in the action part of the rules.

\subsubsection{Element for macro-instruction definition \texttt{<def-macro>}}

Each macro-instruction definition has a mandatory name (the value of the attribute \texttt{n}), the number of arguments it receives (attribute \texttt{npar}) and a body with instructions.

\subsubsection{Element for rules section \texttt{<section-rules>}}

This section contains the structural transfer rules, each one in a \texttt{<\textbf{rule}>} element.

\subsubsection{Element for rule \texttt{<rule>}}

Each rule has a pattern (\texttt{<\textbf{pattern}>}) and the associated action (\texttt{<\textbf{action}>}) performed when the pattern is matched.

The rule can have an optional attribute \texttt{comment} with a comment on, usually, the function of the rule.

\subsubsection{Element for pattern \texttt{<pattern>}}

A pattern is specified using pattern items (\texttt{<\textbf{pattern-\\item}>}), each one of which corresponds to a lexical form in the matched pattern, in order of appearance.

\subsubsection{Element for pattern constituent \texttt{<pattern-item>}}

Each pattern item specifies, in the attribute with mandatory name \texttt{n}, which kind of lexical form is to be matched. To do that, one has to use the categories defined in \texttt{<\textbf{section-def-cats}>} (see in Figure \ref{fig:regla} the definition of a pattern for determiner–noun ).

\subsubsection{Element for action \texttt{<action>}}

This element contains the “instructions” that have to be executed to process as desired each matched pattern.

The processing part for matched patterns is a block of zero or more instructions of the kind: \texttt{<\textbf{choose}>} (conditional processing), \texttt{<\textbf{let}>} (value assignment), \texttt{<\textbf{out}>} (print TL lexical forms), \texttt{<\textbf{modify-case}>} (modify case state of a lexical form), \texttt{<\textbf{call-macro}>} (call a macro-instruction) and \texttt{<\textbf{append}>} (concatenate strings).

Through the processing step, depending on whether a series of conditional options are met or not, different operations are carried out, such as creating agreement between pattern components, necessary when these undergo gender or number changes in the lexical transfer process. To do this, in spite of working with TLLF, also the SL information is taken into account, since, for example, if pattern components do not agree in SL, maybe they do not have to agree in TL either. As a consequence of the application of the different operations in a pattern, values are assigned to pattern attributes and, if applicable, to global or state variables, and the information on the resulting TL pattern is sent to the next module (the morphological generator in a shallow-transfer system, or the next transfer module in an advanced transfer system).

\subsubsection{Element for macro-instruction call \texttt{<call-macro>}}

In a rule it is possible to call any of the macro-instructions defined in \texttt{<\textbf{section-def-macros}>}. To do this, one has to specify the name of the macro-instruction in the \texttt{n} attribute, and one or more arguments in the parameter element \texttt{<\textbf{with-param}>} (see next).

\subsubsection{Element for parameters \texttt{<with-param>}}

This element is used inside a macro-instruction call \texttt{<\textbf{call-macro}>}. The \texttt{pos} attribute of an argument is used to refer to a lexical form of the rule from where the macro-instruction is called. For example, if a macro-instruction with 2 parameters has been defined, to make agreement operations between noun–adjective, it can be used with arguments 1 and 2 in a rule for noun–adjective, with arguments 2 and 3 in a rule for determiner–noun–adjective, with arguments 1 and 3 in a rule for noun–adverb–adjective and with arguments 2 and 1 in a rule for adjective–noun. You can see an example of macro-instruction call in Figure \ref{fig:macro}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{call-macro} n="f_concord2"> <\textbf{with-param} pos="3"/> <\textbf{with-param} pos="1"/> <\textbf{/call-macro}> \end{alltt} \end{small} \caption{Call of the macro-instruction \texttt{f-concord2} designed to create agreement between two elements in a pattern such as determiner–adverb–noun. Propagation of gender and number is done from one of the components, in this case, from the noun which is the third element of the pattern (3). Therefore, the position of the noun is the first parameter given, and the other parameters come next. Since the adverb (in position 2) does not need agreement information, only the position of the determiner is specified (1).} \label{fig:macro} \end{figure}

\subsubsection{Element for selection \texttt{<choose>}} \label{choose}

The selection instruction consists of one or more conditional options (\texttt{<\textbf{when}>}) and an alternative option \texttt{<\textbf{otherwise}>}, which is optional.

\subsubsection{Element for condition \texttt{<when>}}

This element describes a conditional option (see Section \ref{choose}). It contains the condition to be tested \texttt{<\textbf{test}>} and one block of zero or more instructions of the kind \texttt{<\textbf{choose}>}, \texttt{<\textbf{let}>}, \texttt{<\textbf{out}>}, \texttt{<\textbf{modify-case}>}, \texttt{<\textbf{call-macro}>} or \texttt{<\textbf{append}>}, \nota{OK append?} which will be executed if the above condition is met.

\subsubsection{Element for alternative option \texttt{<otherwise>}}

The element \texttt{<\textbf{otherwise}>} contains one block of one or more instructions (of the kind \texttt{<\textbf{choose}>}, \texttt{<\textbf{let}>}, \texttt{<\textbf{out}>}, \texttt{<\textbf{modify-case}>}, \texttt{<\textbf{call-macro}>} and \texttt{<\textbf{append}>}) that must be executed if none of the conditions described in the \texttt{<\textbf{when}>} elements of a \texttt{<\textbf{choose}>} is met.

\subsubsection{Element for evaluation \texttt{<test>}}

The test element \texttt{<\textbf{test}>} in a condition element \texttt{<\textbf{when}>} can contain a conjunction (\texttt{<\textbf{and}>}), a disjunction (\texttt{<\textbf{or}>}) or a negation (\texttt{<\textbf{not}>}) of conditions to be tested, as well as a simple condition of string equality (\texttt{<\textbf{equal}>}), string beginning (\texttt{<\textbf{begins-with}>}), string end (\texttt{<\textbf{ends-with}>}), substring (\texttt{<\textbf{contains-substring}>}) or inclusion in a set (\texttt{<\textbf{in}>}).

\nota{Segur que es pot millorar la redacció de l’últim paràgraf, canviat per mlf perquè hi estiguen totes les condicions booleanes simples.}

\subsubsection{Elements for conditional or boolean operators: \texttt{<equal>}, \texttt{<and>}, \texttt{<or>}, \texttt{<not>}, \texttt{<in>}}

\nota{To be completed: add \texttt{contains-substring}, \texttt{ends-with}, \texttt{begins-with}, etc.}


\item The conjunction element \texttt{<\textbf{and}>} represents a condition, consisting of two or more conditions, that is met when all included conditions are true. An example of its use can be found in Figure \ref{fig:regla}.

\item The disjunction element \texttt{<\textbf{or}>} represents a condition, consisting of two or more conditions, that is met when at least one of the included conditions is true. Figure \ref{fig:ornot} displays an example of this condition type used when testing gender agreement in a SL pattern.

\item The negation element \texttt{<\textbf{not}>} represents a condition that is met when the included condition is not met, and vice versa. An example of negation of an equality can be found in Figure \ref{fig:ornot}.

\item The conditional equality operator \texttt{<\textbf{equal}>} is an instruction that evaluates if two arguments (two strings) are identical or not. See examples of its use in Figures \ref{fig:clip} and \ref{fig:lit-tag}. In addition, this operator can have the attribute \texttt{caseless}, which, when its value is \texttt{yes}, causes the comparison of strings to be made ignoring case. \nota{All string conditional tests have the attribute \texttt{caseless}; also \texttt{in} described below}

\item The "search in lists" operator \texttt{<\textbf{in}>} is used to search for any value (specified as the first parameter of the condition) in a list referred to by the \texttt{n} attribute of the \texttt{<\textbf{list}>} element; this list must be defined in the appropriate section (\texttt{<\textbf{section-def-lists}}). The search result is true if the value is found in the list. This comparison can also use the attribute \texttt{caseless}: if its value is \texttt{yes}, the search is done ignoring case. Figure \ref{fig:in} shows an example of its use.


\nota{Cal unificar tota la discussió anterior, traient factor comú.}

\nota{Cal descriure la resta d’elements condicionals que no hi són.}

\subsubsection{Element \texttt{<clip>}} \label{ss:clip}

The \texttt{<\textbf{clip}>} element represents a substring of a SL or TL lexical form, defined by the value of its different attributes (see an example in Figure \ref{fig:clip}):

\begin{itemize} \item \texttt{pos} is an index (1, 2, 3, etc.) used to select a lexical form inside a rule: it refers to the place the lexical form occupies in the pattern. In the \textit{postchunk} module there is also the index “0”, which refers to the pseudolemma of the chunk \nota{MG: is it not "lexical pseudoform"?}, which is treated as a word by itself in order to be able to consult its information and make decisions from this.

\item \texttt{side} \textit{(only in the \texttt{chunker} module)} specifies if the selected \emph{clip} is from the source language (\texttt{sl}) or from the target language (\texttt{tl}).

\item \texttt{part} indicates which part of the lexical form is processed; generally its value is one of the attributes defined in \texttt{<\textbf{section-def-\\attrs}>} (\texttt{gen}, \texttt{nbr}, etc.), although it can also take four predefined values: \texttt{lem} (refers to the lemma of the lexical form), \texttt{lemh} (the first part of a split lemma), \texttt{lemq} (the queue of a split lemma), and \texttt{whole} (the whole lexical form, including lemma and all grammatical symbols, which may have been modified in the preceding part of the rule).

\item \texttt{link-to} \textit{(only in the \texttt{chunker} module in advanced mode)} replaces the value that would result from consulting the rest of the attributes of the clip, by the value specified in this attribute, which must be a natural number ($>0$). \nota{MG: explain the new characteristics - Sergio?} This number indicates to which \texttt{<\textbf{tag}>} of the \texttt{<\textbf{chunk}>} is linked the clip content, the number being the order this tag occupies inside the element \texttt{<\textbf{tags}>}. The other attributes of the clip remain only for informational purposes, since they are overwritten by the value of the linked tag. An example of its use can be found in Figure \ref{fig:chunkintrachunk}.


\begin{figure} \begin{small} \begin{alltt} <\textbf{test}> <\textbf{not}> <\textbf{equal}> <\textbf{clip} \textsl{pos}="2" \textsl{side}="tl" \textsl{part}="gen"/> <\textbf{clip} \textsl{pos}="2" \textsl{side}="sl" \textsl{part}="gen"/> <\textbf{/equal}> <\textbf{/not}> <\textbf{/test}> \end{alltt} \end{small} \caption{Extract from a rule where it is tested whether the TL (\texttt{tl}) gender (\texttt{gen}) of the second lexical unit identified in a pattern is different from the gender of the same lexical unit in the SL (\texttt{sl})}. \label{fig:clip} \end{figure}

\subsubsection{Element for literal string \texttt{<lit>}} This element is used to specify the value of a literal string by means of the attribute \texttt{v}. For example, \texttt{<\textbf{lit} v=\texttt{"}andar\texttt{"}/>} represents the string \emph{andar}.

\subsubsection{Element for tag value \texttt{<lit-tag>}} It is similar to the \texttt{<\textbf{lit}>} element, with the difference that it does not specify the value of a literal string but the value of a grammatical symbol or tag, by means of the attribute \texttt{v}. An example of its use can be found in Figure \ref{fig:lit-tag}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{equal}> <\textbf{clip} \textsl{pos}="2" \textsl{side}="tl" \textsl{part}="nbr"/> <\textbf{lit-tag} \textsl{v}="ND"/> <\textbf{/equal}> \end{alltt} \end{small} \caption{Use of the element \texttt{<\textbf{lit-tag}>}: it is tested whether the number (\texttt{nbr}) symbol of the second lexical unit in the TL (\texttt{tl}) is \texttt{ND} (number to be determined)} \label{fig:lit-tag} \end{figure}

\begin{figure} \begin{small} \begin{alltt} <\textbf{test}> <\textbf{or}> <\textbf{not}> <\textbf{equal}> <\textbf{clip} \textsl{pos}="1" \textsl{side}="sl" \textsl{part}="gen"/> <\textbf{clip} \textsl{pos}="3" \textsl{side}="sl" \textsl{part}="gen"/> <\textbf{/equal}> <\textbf{/not}> <\textbf{not}> <\textbf{equal}> <\textbf{clip} \textsl{pos}="2" \textsl{side}="sl" \textsl{part}="gen"/> <\textbf{clip} \textsl{pos}="3" \textsl{side}="sl" \textsl{part}="gen"/> <\textbf{/equal}> <\textbf{/not}> <\textbf{/or}> <\textbf{/test}> \end{alltt} \end{small} \caption{Extract from a rule where it is tested whether the SL gender of the first or the second lexical unit matched in a pattern (it could be, for example, determiner–adjective–noun) is different from the gender of the third lexical unit also in the SL.} \label{fig:ornot} \end{figure}

\subsubsection{Element for variable \texttt{<var>}}

Each \texttt{<\textbf{var}>} is a variable identifier: the mandatory attribute \texttt{n} specifies its name as has been defined in \texttt{<\textbf{section-def-vars}>}. When it appears in an \texttt{<\textbf{out}>}, a \texttt{<\textbf{test}>}, or the right part of a \texttt{<\textbf{let}>}, it represents the value of the variable; when it appears on the left side of a \texttt{<\textbf{let}>}, in an \texttt{<\textbf{append}>} or in a \texttt{<\textbf{modify-case}>}, it represents the reference of the variable and its value can be changed.

\subsubsection{Element for reference to string list \texttt{<list>}}

This element is only used as the second parameter of a \texttt{<\textbf{in}>} search. The \texttt{n} attribute refers to the specific list defined in the string lists definition section \texttt{<\textbf{section-def-lists}>}. An example of its use can be found in Figure \ref{fig:in}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{rule}> <\textbf{pattern}> <\textbf{pattern-item} \textsl{n}="verb"/> <\textbf{pattern-item} \textsl{n}="a"/> <\textbf{/pattern}> <\textbf{action}> <\textbf{choose}> <\textbf{when}> <\textbf{test}> <\textbf{in} \textsl{caseless}="yes"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="sl" \textsl{part}="lem"/> <\textbf{list} \textsl{n}="verbos_est"/> <\textbf{/in}> <\textbf{/test}> <\textbf{let}> <\textbf{clip} \textsl{pos}="2" \textsl{side}="tl" \textsl{part}="lem"/> <\textbf{lit} \textsl{v}="en"/> <\textbf{/let}> <\textbf{/when}> <!– ... –> \end{alltt} \end{small} \caption{Extract of a rule that detects a pattern made of a verb and the preposition \emph{a}, and then testes whether the verb (the lemma indicated in \texttt{lem}) of the source language (\texttt{sl}) is one of the lemmas included in the list of state verbs (defined in Figure \ref{fig:deflist}). If that be the case, the lemma of the second word in target language (\texttt{tl}) is changed to \emph{en}.} \label{fig:in} \end{figure}

\subsubsection{Element for case application \texttt{<get-case-from>}}

The \texttt{<\textbf{get-case-from}>} element represents the string obtained after applying the letter case state of the lemma of a SL lexical unit to a string (\emph{clip}, \emph{lit} or \emph{var}). To refer to the lexical unit from where the information is taken, the attribute \texttt{pos} is used, which indicates the position of that unit in the SL. This element is useful when the lexical units in a pattern are reordered, or when a lexical unit is added or deleted. You can see an example of its use in Figure \ref{fig:case}, which displays a rule to transform the simple perfect preterite tense in Spanish (\emph{dije}, "I said") into the compound form in Catalan (\emph{vaig dir}). In this rule, a LF with lemma \emph{anar} and grammatical symbol \emph{vaux} ("auxiliary verb") is added; it has to take the case information from the Spanish verb (which has position "1" in the pattern), so that the system translates \emph{Dije} as \emph{Vaig dir}, \emph{dije} as \emph{vaig dir} and \emph{DIJE} as \emph{VAIG DIR}.

\subsubsection{Element for case pattern query \texttt{<case-of>}}

It is used to get the case pattern of a string, that is, one of the values "\texttt{aa}", "\texttt{Aa"} or "\texttt{AA}". It works like the \texttt{<\textbf{clip}>} element, since it has the same attributes: \texttt{pos}, the position of the word in the matched pattern; \texttt{part}, the specific attribute that we refer to (normally the lemma), which has the predefined attributes described in Section \ref{ss:clip}, and finally, only in the \texttt{chunker} module, the attribute \texttt{side}, referring to the translation side, \texttt{sl} or \texttt{tl}. In Figure \ref{fig:case} you can see this element in use, and you can find a more detailed description of this example in the following Section (description of \texttt{<\textbf{modify-case}>}).

\subsubsection{Element for case modification \texttt{<modify-case>}}

This instructions is used to modify the case of the first parameter (usually a lemma) by means of the second parameter (a literal or a variable). The first parameter can be a \texttt{<\textbf{var}>}, a \texttt{<\textbf{clip}>} or a \texttt{<\textbf{case-of}>}, whereas the second one can be anything that delivers a value, but in principle it will be a \texttt{<\textbf{var}>} or a \texttt{<\textbf{lit}>}. The values that this value can take are usually “\texttt{Aa}”, to express that the “left part” of this case modification must have the first letter in upper case and the rest in lower case, “\texttt{aa}” to put all in lower case, and “\texttt{AA}” to put all in upper case.

Figure \ref{fig:case} shows a rule where this element is used. It modifies in this rule the case of the TL lemma in position "1", which corresponds to \emph{dir}, because, although in the rule output this verb is the second lexical form (\emph{vaig dir}), it is actually the translation of the LF which has position 1 in the SL, and, therefore, it retains the same assigned position in the TL. This lemma is assigned the value “\texttt{aa}” in the case that the SL lemma has the state “\texttt{Aa}”. There is nothing to specify for the rest of the cases, since the case state of the LF in position 1 will be the same in the SL and in the TL and, therefore, will be automatically transferred (see Section~\pageref{mayusc} to obtain more information on letter case handling in dictionaries ).

\begin{figure} \begin{small} \begin{alltt} <\textbf{rule}> <\textbf{pattern}> <\textbf{pattern-item} n="pretind"/> <\textbf{/pattern}> <\textbf{action}> <\textbf{out}> <\textbf{lu}> <\textbf{get-case-from} pos ="1"> <\textbf{lit} v="anar"/> <\textbf{/get-case-from}> <\textbf{lit-tag} v="vaux"/> <\textbf{clip} pos="1" side="sl" part="persona"/> <\textbf{clip} pos="1" side="sl" part="nbr"/> <\textbf{/lu}> <\textbf{b/}> <\textbf{/out}> <\textbf{choose}> <\textbf{when}> <\textbf{test}> <\textbf{equal}> <\textbf{case-of} pos="1" side="sl" part="lemh"/> <\textbf{lit} v="Aa"/> <\textbf{/equal}> <\textbf{/test}> <\textbf{modify-case}> <\textbf{case-of} pos="1" side="tl" part="lemh"/> <\textbf{lit} v="aa"/> <\textbf{/modify-case}> <\textbf{/when}> <\textbf{/choose}> <\textbf{out}> <\textbf{lu}> <\textbf{clip} pos="1" side="tl" part="lemh"/> <\textbf{clip} pos="1" side="tl" part="a_verb"/> <\textbf{lit-tag} v="inf"/> <\textbf{clip} pos="1" side="tl" part="lemq"/> <\textbf{/lu}> <\textbf{/out}> <\textbf{/action}> <\textbf{/rule}> \end{alltt} \end{small} \caption{Rule for the translation from Spanish into Catalan, which turns the verbs in simple perfect preterite tense (\emph{dije}) into the compound perfect preterite tense usual in Catalan (\emph{vaig dir}), and at the same time assigns the appropriate case state to the two resulting words.} \label{fig:case} \end{figure}

\subsubsection{Element for assignment \texttt{<let>}}

The assignment instruction \texttt{<\textbf{let}>} assigns the value of the right part of the assignment (a literal string, a \texttt{clip}, a variable, etc.) to the left part (a \texttt{clip}, a variable, etc.). An example of its use can be found in Figure \ref{fig:regla}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{rule}> <\textbf{pattern}> <\textbf{pattern-item} n="det"/> <\textbf{pattern-item} n="nom"/> <\textbf{/pattern}> <\textbf{action}> <\textbf{choose}> <\textbf{when}> <\textbf{test}> <\textbf{and}> <\textbf{not}> <\textbf{equal}> <\textbf{clip} pos="2" side="tl" part="gen"/> <\textbf{clip} pos="2" side="sl" part="gen"/> <\textbf{/equal}> <\textbf{/not}> <\textbf{not}> <\textbf{equal}> <\textbf{clip} pos="2" side="tl" part="gen"/> <\textbf{lit-tag} v="mf"/> <\textbf{/equa}l> <\textbf{/not}> <\textbf{not}> <\textbf{equal}> <\textbf{clip} pos="2" side="tl" part="gen"/> <\textbf{lit-tag} v="GD"/> <\textbf{/equal}> <\textbf{/not}> <\textbf{/and}> <\textbf{/test}> <\textbf{let}> <\textbf{clip} pos="1" side="tl" part="gen"/> <\textbf{clip} pos="2" side="tl" part="gen"/> <\textbf{/let}> <\textbf{/when}> <\textbf{/choose}> <!– Other gender and number agreement actions –> \end{alltt} \end{small} \caption{Extract from a rule for the pattern \texttt{determiner–noun} (continues in Fig. \ref{fig:regla2}): in this part of the rule, the gender of the noun is assigned to the determiner in the case that the gender of the noun changes from the SL (\texttt{sl}) to the TL (\texttt{tl}) during the lexical transfer process between both languages.} \label{fig:regla} \end{figure}

\subsubsection{Element for string concatenation \texttt{<concat>}}

This element is used to concatenate strings in order to assign them to a variable. It is used in combination with \texttt{<\textbf{let}>}, and the previous value of the variable is lost with the assignment of \texttt{<\textbf{concat}>}.

It does not have any attribute. It can contain any instruction that delivers a string, such as \texttt{<\textbf{lit}>}, \texttt{<\textbf{lit-tag}>} or \texttt{<\textbf{clip}>}.

Figure \ref{fig:concat} shows an example of its use.

\begin{figure} \begin{small} \begin{alltt} <\textbf{let}> <\textbf{var} n="palabra"/> <\textbf{concat}> <\textbf{clip} pos="3" side="tl" part="lem"/> <\textbf{lit-tag} v="adj"/> <\textbf{/concat}> <\textbf{/let}> \end{alltt} \end{small} \caption{In this example, the variable \texttt{palabra} is assigned the value of the concatenation of a \texttt{clip} (the lemma in position 3) and the \emph{adj} tag.} \label{fig:concat} \end{figure}

\subsubsection{Element for string concatenation \texttt{<append>}}

The \texttt{<\textbf{append}>} instruction can be used to save the output of an action before printing it in the corresponding \texttt{<\textbf{out}>}, if required by the designer of the transfer rules.

The mandatory attribute \texttt{n} specifies the name of the variable used. After applying the instruction, the previous content of the referred variable will be the prefix of the new content, that is, the new content inserted in the \texttt{<\textbf{append}>} will be concatenated to the pre-existing content of the variable specified in \texttt{n}.

The content of this instruction can be one or more of the following tags: \texttt{<\textbf{b}>}, \texttt{<\textbf{clip}>}, \texttt{<\textbf{lit}>}, \texttt{<\textbf{lit-tag}>}, \texttt{<\textbf{var}>}, \texttt{<\textbf{get-case-from}>}, \texttt{<\textbf{case-of}>} or \texttt{<\textbf{concat}>}. There is an example of its use in Figure \ref{fig:append}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{append} n="temporal"> <clip pos="3" part="gen" side="tl"/> <\textbf{/append}> \end{alltt} \end{small} \caption{In this example, the variable \texttt{temporal} is assigned the value of the gender, in the TL, of the third word matched by the rule.} \label{fig:append} \end{figure}

\subsubsection{Element for output \texttt{<out>}}

\label{ss:out} The output instruction is used to specify the lexical forms that are sent at the output of the module after having been applied the required structural transfer operations. Its use is different according to the module. On the one hand, its use in the \texttt{chunker} module when it runs as only module (shallow-transfer) and its use in the \texttt{postchunk} module are similar, since in both cases, the output must be the input for the generator. The \texttt{chunker} in Apertium 2 and the \texttt{interchunk} have different use modes: the former to create the chunks, and the latter to modify the chunks without modifying its internal part.


\item \textbf{Use in \texttt{chunker} in shallow-transfer mode, and in \texttt{postchunk}}

The instruction sends each lexical form inside a \texttt{<\textbf{lu}>} set, which in turn can be contained inside a \texttt{<\textbf{mlu}>} element when the output is a multiword made of two or more LF. Besides, also the blanks or superblanks (\texttt{<\textbf{b}>}) between LF and LF are sent. You can find an example of its use in Figures \ref{fig:case} and \ref{fig:regla2}.

\begin{figure} \begin{small} \begin{alltt} <!– ... –> <\textbf{out}> <\textbf{lu}> <\textbf{clip} pos="1" side="tl" part="whole"/> <\textbf{/lu}> <\textbf{lu}> <\textbf{clip} pos="2" side="tl" part="whole"/> <\textbf{/lu}> <\textbf{/out}> <\textbf{/process}> <\textbf{/action}> <\textbf{/rule}> \end{alltt} \end{small} \caption{Extract from a rule (comes from Fig. \ref{fig:regla}). At the end of the rule, and after different actions, the resulting data are sent by means of the attribute \texttt{whole}, which contains the lemma and the grammatical symbols of each LF (positions 1 and 2 in the pattern).} \label{fig:regla2} \end{figure}

\item \textbf{Use in \texttt{chunker} in advanced mode}

The output of this module is expected to be a sequence of one or more chunks (sent inside a \texttt{<\textbf{chunk}>} element) separated by blanks \texttt{<\textbf{b}>}. Lexical forms and multiforms, as well as the blanks between them, are sent inside chunks. You can see in Figure \ref{fig:chunkintrachunk} an example of use.

\begin{figure} \begin{small} \begin{alltt} <\textbf{out}> <\textbf{chunk} name="pr" case="caseFirstWord"> <\textbf{tags}> <\textbf{tag}><\textbf{lit-tag} v="PREP"/><\textbf{/tag}> <\textbf{/tags}> <\textbf{lu}> <\textbf{clip} pos="1" side="tl" part="whole"/> <\textbf{/lu}> <\textbf{/chunk}> <\textbf{b} pos="1"/> <\textbf{chunk} name="probj" case="caseOtherWord"> <\textbf{tags}> <\textbf{tag}><\textbf{lit-tag} v="NP"/><\textbf{/tag}> <\textbf{tag}><\textbf{lit-tag} v="tn"/><\textbf{/tag}> <\textbf{tag}><\textbf{clip} pos="2" side="tl" part="pers"/><\textbf{/tag}> <\textbf{tag}><\textbf{clip} pos="2" side="tl" part="gen"/><\textbf{/tag}> <\textbf{tag}><\textbf{clip} pos="2" side="tl" part="nbr"/><\textbf{/tag}> <\textbf{/tags}> <\textbf{lu}> <\textbf{clip} pos="2" side="tl" part="lem"/> <\textbf{lit-tag} v="prn"/> <\textbf{lit-tag} v="2"/> <\textbf{clip} pos="2" side="tl" part="pers"/> <\textbf{clip} pos="2" side="tl" part="gen" link-to="4"/> <\textbf{clip} pos="2" side="tl" part="nbr" link-to="5"/> <\textbf{/lu}> <\textbf{/chunk}> <\textbf{/out}> \end{alltt} \end{small} \caption{Output instruction that sends two chunks separated by a blank. The printed sequence is a preposition followed by a noun phrase ("NP"). The tags that are linked from the second chunk to the outside are pronoun type ("tn"), gender and number of the noun phrase (pronoun). The \texttt{<\textbf{tag}>} elements are used to specify the tags of the chunk, and the value of the attributes \texttt{name} and \texttt{case} is used to specify the pseudolemma of the chunk.} \label{fig:chunkintrachunk} \end{figure}

\item \textbf{Use in \texttt{interchunk}}

In this module, lexical forms (words) are inaccessible, since it is only possible to operate with chunks and, therefore, inside an \texttt{<\textbf{out}>} element you can only put \texttt{<\textbf{chunk}>} elements or blanks \texttt{<\textbf{b}>}. The information on lemma and tags specified here in a \texttt{<\textbf{chunk}>} element refers exclusively to the lemma (pseudolemma) and the tags of the chunk.

An example of its use can be found in Figure \ref{fig:chunkinterchunk}.

\begin{figure} \begin{small} \begin{alltt} <\textbf{out}> <\textbf{b} pos="1"/> <\textbf{chunk}> <\textbf{clip} pos="2" part="lem"/> <\textbf{clip} pos="2" part="tags"/> <\textbf{clip} pos="2" part="chcontent"/> <\textbf{/chunk}> <\textbf{/out}> \end{alltt}

\end{small} \caption{The aim of this rule output is to discard the first chunk of the matched pattern (pronoun drop). The three \texttt{<\textbf{clip}>} elements have been included here for illustrative purposes, since they could have been replaced by the \texttt{part="whole"} which would group them in a single \texttt{<\textbf{clip}>} .} \label{fig:chunkinterchunk} \end{figure}


\subsubsection{Element for lexical unit \texttt{<lu>}}

\label{ss:lu} This is the element by means of which each TLLF is sent out at the end of a rule, inside an \texttt{<\textbf{out}>} element. With this element, one can send the whole lexical form, using the attribute \texttt{whole} of a \texttt{<\textbf{clip}>}, or, if required, specify its parts separately (lemma plus tags, indicated by means of \texttt{<\textbf{clip}>} strings, literal strings \texttt{<\textbf{lit}>}, tags \texttt{<\texttt{\textbf{lit-tag}}>}, variables \texttt{<\texttt{\textbf{var}}>}, besides case information [\texttt{<\textbf{get-case-from}>}, \texttt{<\textbf{case-of}>}]).

Please note that, as has been explained before, in the case of multiwords with \emph{split lemma} it is necessary to replace the lemma queue \emph{after} the grammatical symbols of the inflected word (or lemma head), because the \texttt{pretransfer} module has moved the queue to put it before the grammatical symbols of the head. This replacement is done here, inside the \texttt{<\textbf{lu}>} element, using the values \texttt{lemh} and \texttt{lemq} of the attribute \texttt{part} in a \texttt{<\textbf{clip}>}. The \texttt{lemh} attribute refers to the lemma head, and \texttt{lemq} to the lemma queue. As can be seen in the example \ref{fig:case}, the \texttt{lemq} part of a \texttt{<\textbf{clip}>} is placed after the lemma head and all the grammatical symbols that follow it. This rule would be suitable, for example, for the Spanish form \emph{eché de menos} ("I missed"), which has to be translated into Catalan as \emph{vaig trobar a faltar}. The attribute \texttt{a\_verb} which comes after \texttt{lemh} contains the grammatical symbol that describes the verb category (\emph{vblex}, \emph{vbser}, \emph{vbhaver} or \emph{vbmod} as applicable). Therefore, the last lexical form sent by this rule, in the case of \emph{vaig trobar a faltar}, would be, in the data stream: \begin{alltt} ^trobar<vblex><inf># a faltar\$ \end{alltt}

\noindent The number sign \texttt{\#} in the data stream corresponds to the \texttt{<\textbf{g}>} element in dictionaries, used to signal the position of the invariable part in a split lemma multiword.

It is important to note that the attributes included in \texttt{<\textbf{lu}>} may be empty. So, a verb matched by the rule in Fig. \ref{fig:case} which is not a split lemma multiword, will be sent with an empty \texttt{lemq} attribute, since the verb does not have lemma queue. This way it is not necessary to define different rules for lexical forms with and without queue. You can find another example of this in page \pageref{regla_verbo1}, where the rule for verb sends in a \texttt{<\textbf{lu}>} the attributes \texttt{gen} (\emph{gender}) and \texttt{nbr} (\emph{number}). This way, it includes participles (with gender and number) and the rest of verb forms (which will have these attributes empty).

In the same page you can see a rule for a verb followed by an enclitic pronoun. Here, the lemma queue is placed after the enclitic pronoun; so, for a split lemma multiword joined to an enclitic pronoun, such as \emph{echándote de menos}, the output in the data stream would be, when translating into Catalan:

\begin{alltt} ^trobar<vblex><ger>+et<prn><enc><p2><mf><sg># a faltar\$ \end{alltt}

Of course, this rule works also for verbs which do not belong to this multiword type; so, the form \emph{explicándote} ("explaining to you") would be output, when translating from Spanish to Catalan:

\begin{alltt} ^explicar<vblex><ger>+et<prn><enc><p2><mf><sg>\$ \end{alltt}

As for the attribute \texttt{whole} of a \texttt{<\textbf{clip}>}, it must be taken into account that it can be used to send the whole lexical form only in the case that the sent word can not be a multiword, that is, can not contain a split lemma. Compare figures \ref{fig:case} and \ref{fig:regla2}. The \texttt{whole} attribute can be used in the second example because it contains the lemma \texttt{lem} plus all the morphological tags of the lexical forms in position 1 and 2 (determiner and noun). \nota{but nouns can also be mw now!}Contrarily, in the first example, the lexical form in \texttt{<\textbf{lu}>} is sent in parts, with a \texttt{lemh} (lemma head) and a \texttt{lemq} (lemma queue), since it may occur that the verb matched in the pattern is a multiword with split lemma. In practice, in our system this means that the \texttt{whole} attribute can be used to send any kind of lexical form except verbs and nouns, because we defined multiwords with inner inflection only for verbs and nouns.

\subsubsection{Element for lexical unit \texttt{<mlu>}} \label{ss:mlu}

Its name derives from \emph{multilexical unit}; it is used inside the \texttt{<\textbf{out}>} element to output multiwords consisting of more than one lexical form. Each lexical form in a \texttt{<\textbf{mlu}>} is sent inside a \texttt{<\textbf{lu}>} element. On the output of the module, lexical forms contained in this element will be joined to each other by the symbol ’+’ in the data stream. This means that they will become a multiword made of different lexical forms, which will be treated as a single unit by the subsequent modules; therefore, the generation dictionary will have to contain an entry for this multiword in order for it to be generated.

In our system, this element is used to join enclitic pronouns to conjugated verbs.

\subsubsection{Element for chunk encapsulation \texttt{<chunk>}}

This is the element in which chunks are sent, in an \texttt{<\textbf{out}>} element, on the output of the module. It is only used in the \texttt{chunker} module in advanced mode, and in the \texttt{interchunk} module. It is not used in the \texttt{postchunk} module because its output does not contain any chunk. Neither it is used in the \texttt{chunker} module in shallow-transfer mode, because its output does not contain chunks but individual lexical units and blanks.


\item \textbf{Use in \texttt{chunker} in advanced mode}

In this mode, the \texttt{<\textbf{chunk}>} element must have an attribute \texttt{name}, which is the lemma of the chunk, or an attribute \texttt{namefrom} which refers to a variable previously defined, whose value will be used as the lemma of the chunk. Besides, it can include the attribute \texttt{case} to specify which variable is the case policy taken from (for example, a value obtained with the instruction \texttt{<\textbf{case-from}>}).

An example of its use can be found in Figure \ref{fig:chunkintrachunk}.

\item \textbf{Use in \texttt{interchunk}}

In this module, the \texttt{<\textbf{chunk}>} element does not specify any attribute; it is used just as the \texttt{<\textbf{lu}>} element is used in the shallow-transfer or in the \texttt{postchunk} to delimit the lexical forms. The elements it sends are (generally in a \texttt{<\textbf{clip}>} instruction): the lemma of the chunk (\texttt{lem}), its tags (\texttt{tags}) and the chunk content (\texttt{chcontent}, contains LF plus blanks), which is an invariable part since it can not be accessed from the \texttt{interchunk} module. The invariable part of the chunk is sent at the end. You can also use the \texttt{whole} attribute to send the whole chunk (lemma, tags and invariable content).

An example of its use can be found in Figure \ref{fig:chunkinterchunk}.


\subsubsection{Element for tag links section \texttt{<tags>}}

\textit{Only in chunker in advanced mode}.

This element is used to specify a list of tags, or \texttt{<\textbf{tag}>} elements, which will become the pseudotags of the chunk. It does not have attributes, and must be included as first item inside the \texttt{<\textbf{chunk}>} element. See Figure \ref{fig:chunkintrachunk}.

\subsubsection{Element for tag link \texttt{<tag>}}

\textit{Only in chunker in advanced mode}.

The \texttt{<\textbf{tag}>} element must contain a morphological tag, which can be specified by means of a \texttt{<\textbf{clip}>} instruction or a literal tag \texttt{<\textbf{lit-tag}>}. It does not have attributes.

The tag or tags specified this way in a chunk will become the grammatical symbols of the chunk; the next module, \texttt{interchunk}, will be able to use them to test and modify the characteristics of the chunks.

\subsubsection{Element for blank \texttt{<b>}}

The \texttt{<\textbf{b}>} element refers to [super]blanks and is indexed by the attribute \texttt{pos}. For example, a \texttt{<\textbf{b}>} with \texttt{pos="2"} refers to the [super]blanks (including format data encapsulated by the de-formatter) between the 2nd SLLF and the 3rd SLLF. The explicit management of [super]blanks enables the correct placement of format when the result of the structural transfer has more or less elements than the original, or when it has been reordered in some way.

\subsection{Specification of the three modules that build an advanced transfer system} \label{noutransfer}

In the following lines we describe the differences between the rule format in the three modules of an advanced transfer system. When Apertium works as a shallow-transfer system, the only module to be run is the first one, called \texttt{chunker}, which communicates directly with the generation module.

\subsubsection{\texttt{Chunker} module} \label{ss:chunker}

This module can be used alone as a shallow-transfer system, or in combination with the other two transfer modules to build an advanced transfer system. An attribute of the \texttt{<transfer>} element controls its run mode.


\begin{itemize} \item Input: data in the \texttt{pretransfer} output format, that is, with invariable queues of multiwords moved to the position right before the first grammatical symbol.

\item Output: \begin{itemize} \item[-] in advanced mode (in an advanced transfer system): chunks, that will be detected and processed by the next module \item[-] in shallow-transfer mode (in a shallow-transfer system): lexical forms, that will be the input of the generation module. \end{itemize}


\paragraph{Data files}

\nota{Explicar millor això de l’únic fitxer de configuració}

This program uses a single configuration file and a precompiled file for pattern detection calculated from the former. The name of the pattern file (the configuration file) will have the extension \texttt{.t1x}. Since the \texttt{chunker} is the program that looks up the bilingual dictionary, this dictionary (compiled) also has to be provided to the program.

\nota{Potser seria bona idea esmentar en quina secció s’explica el compilador a què es fa referència}

The DTD of this data file is specified in Appendix \ref{ss:dtdtransfer}, and the elements used to create the rules in the file are described in Section \ref{formatotransfer}.

\paragraph{Pattern matching}

The rule matching system in this module will be the one described in \ref{functransfer}, since it is the same in advanced transfer mode and in shallow-transfer mode. The \texttt{a\-per\-tium-pre\-trans\-fer} program \nota{Vacil·lació terminològica \texttt{pretransfer}.} is needed to adapt the tagger output format to the input format required by the transfer module. There is the possibility that, in later versions of Apertium, the \textit{part-of-speech tagger} is modified so that it does the work of \texttt{apertium-pretransfer}. \nota{També hem d’unificar la terminologia d’altres mòduls: \emph{desambiguador categorial}, \emph{etiquetador}; tal com està redactat el paràgraf es podria pensar que són dues coses diferents.}

\paragraph{How it works}

The module works similarly in shallow-transfer mode and in advanced mode, with these differences:

\begin{itemize} \item If we want that the module works as the first module in an advanced transfer system, we must specify the value \texttt{chunk} in the optional attribute \texttt{default} of the root element \texttt{<transfer>}. The default value is \texttt{lu}, which implies that the \texttt{chunker} works in shallow-transfer mode (as a single module).

\item Chunk generation in the output: the \texttt{<chunk>} tag is an element one level higher than \texttt{<lu>} (\textit{lexical unit}), which generates chunks with the characteristics described in \ref{sec:format}; it has the following attributes:

\begin{itemize} \item \texttt{name} (optional): pseudolemma of the chunk. It contains a string that is identified as the pseudolemma of the chunk.

\item \texttt{namefrom} (optional): pseudolemma of the chunk, obtained from a variable. It is compulsory to specify whether \texttt{name} or \texttt{namefrom}.

\item \texttt{case} (optional): variable that is used to obtain the information on case from it and apply it to the lemma specified in \texttt{name} or in \texttt{namefrom}. \end{itemize}

\item Each chunk begins with a \texttt{<tags>} instruction, which does not allow any attribute, and which contains one or more individual instructions \texttt{<tag>}. \item Instructions \texttt{<tag>} do not have attributes. They can include any instruction that returns a string as a value: \texttt{<lit>}, \texttt{<var>} \nota{clip, lit-tag}. \item Instructions \texttt{<clip>} have an optional attribute: \texttt{link-to}, which is used to specify a tag \verb!<!\textit{value of link-to}\verb!>! that replaces \nota{Spanish: “una etiqueta en lugar de” (instead of) or “additionally”?. Explain new aspects of link-to} the information specified by the \texttt{<clip>} in the rest of its attributes.\nota{No s’entén gaire bé - Not understandable} This information is dispensable but can be useful as information on the origin of the linguistic decision. \end{itemize}

The following is a use example of the \texttt{<chunk>} element :

\begin{alltt} <out> <chunk name="adj-noun" case="variableCase"> <tags> <tag><lit-tag v="NP"/></tag> <tag><clip pos="2" side="tl" part="gen"/></tag> <tag><clip pos="2" side="tl" part="nbr"/></tag> </tags> <lu> <clip pos="2" side="tl" part="lemh"/> <clip pos="2" side="tl" part="a_noun"/> <clip pos="2" side="tl" part="gen" link-to="2"/> <clip pos="2" side="tl" part="nbr" link-to="3"/> </lu> <b pos="1"/> <lu> <var n="adjectiu"/> <clip pos="1" side="tl" part="lem"/> <clip pos="1" side="tl" part="a_adj"/> <clip pos="2" side="tl" part="gen" link-to="2"/> <clip pos="2" side="tl" part="nbr" link-to="3"/> </lu> </chunk> </out> \end{alltt}

\paragraph{Default action}

Isolated \textit{superblanks} which are not detected by any pattern in this module, are written in the same order in which they arrive.

The default action for words not matched by any pattern is different depending on the transfer mode (that is, on the value of the optional attribute \texttt{default} of the root element \texttt{<transfer>}):

\begin{itemize} \item if the value is \texttt{chunk} (i.e. the module works in advanced mode): it will generate trivial chunks with the words not matched by any rule, so that in the output there are no words not included in a chunk. The new chunk will be created with the translation of the word by the bilingual dictionary. The fixed lemma of these implicitly created chunks is \texttt{default}. \item if the value is \texttt{lu} (default value; i.e. the module works as single module in a shallow-transfer system): it will not create chunks for words not matched by rules, they will just be translated using the bilingual dictionary.


The following is an automatically generated chunk for a lexical form not matched by any rule in the \texttt{chunker} module when the \texttt{default} attribute has the value \texttt{chunk}:

\begin{alltt} ^default\verb!{!^that<cnjsub>$\verb!}!$ \end{alltt}

\nota{Va sense etiquetes entre \texttt{default} i \texttt{\{}? No caldria dir-ho explícitament?}

\subsubsection{\texttt{Interchunk} module} \label{ss:interchunk}

\nota{\texttt{apertium-interchunk} or simply \texttt{interchunk}?}

The \texttt{interchunk} module processes chunks; it may reorder them and change its morphosyntactic information. This is done by detecting patterns of chunks (sequences of chunks). The instructions that control how it works are, with little differences, the same used by the \texttt{chunker} module; they are written, however, in a different file. Chunks are processed here in a similar way as words are processed in the \texttt{chunker} of Apertium. \nota{Comprovar la denominació dels programes}


\begin{itemize} \item Input: chunks from the \texttt{chunker}. \item Output: chunks possibly reordered and with the data on its pseudolemmas (lexical pseudoforms) possibly changed. \end{itemize}

\paragraph{Data files}

This module uses two data files. A specification file of the \texttt{in\-ter\-chunk} program, with extension \texttt{.t2x} by analogy with the previous module, and a file of precalculated patterns to accelerate the analysis of the input. The binary file of the bilingual dictionary is not included because it is not used. \nota{Citar el compilador?}

The syntax of the specification file is very similar to that of the \texttt{chunker}. Its DTD is specified in Appendix \ref{ss:dtdinterchunk}, and the elements used to create the rules in the file are described in Section \ref{formatotransfer}.

\paragraph{Pattern matching}

Rules detect patterns defined by sequences of lexical pseudoforms. These lexical pseudoforms have a format based on the format of lexical forms for words. In practice, a lexical pseudoform is seen equivalently as \nota{mlforcada: La alternança \emph{pseudolema} i \emph{pseudoparaula} s’ha de resoldre. MG: ho he traduit tot com a ’lexical pseudoform’, crec que era aquest el sentit.} lexical forms are seen in the \texttt{chunker} regarding pattern matching. This way, pattern matching will be based on attributes defined for lexical pseudoforms, not for lexical forms (words) of the original pattern.

\paragraph{How it works}

With regard to the set of instructions used in \texttt{chunker}, the changes on the set of instructions for this module are the following:

\begin{itemize} \item The root element is called \texttt{<interchunk>} and does not have any attribute. \item The attribute \texttt{side} disappears: This module does not use bilingual dictionaries; therefore, the attribute used to indicate whether the consulted side is SL or TL looses sense. This attribute was basically used in the \texttt{<out>} instructions. \item The \texttt{<chunk>} tag is used here without attributes, simply inside an \texttt{<out>} to delimit the output of chunks. \item The predefined attribute \texttt{lem} refers to the pseudolemma of the chunk. In the same way, the predefined attribute \texttt{tags} refers to the grammatical symbols or tags of the chunk. The chunk content becomes something like a queue which can be printed with the implicit attribute \texttt{chcontent}.\nota{Només imprimir o s’hi pot fer referència també?} \nota{Dir de quin element són aquests atributs} \item All the values of \texttt{part}, except \texttt{chname}, access the pseudolemma and the tags of the chunk (not of individual words). \item Unlike what happens in the \texttt{chunker} module, in the rules of this module it is not allowed to print anything else than \texttt{<chunk>}s in the \texttt{<out>} instructions, in no case isolated words.\nota{MG: and blanks too, right?} \end{itemize}

\paragraph{Default action}

Like in the previous module, a default action has been defined, which writes without modifications the chunks not matched by any pattern of the specification file. This default action writes exactly what it reads, be it chunks or blanks. \nota{Atenció a la vacil·lació \emph{regla}/\emph{acció} en la resta del document. Sempre havia cregut que era \emph{regla}=\emph{patró}+\emph{acció}.}

\subsubsection{\texttt{Postchunk} module} \label{ss:postchunk}

The \texttt{postchunk} module detects single chunks and, for each of them, performs the specified actions. Detection is based on the lemma of the chunk, and not in patterns (not in tags); this causes detection in this module to be done specific for each “name” of chunk.\nota{Quan fixem bé la terminologia hem d’assegurar-nos que la redacció d’aquesta part és l’adequada.}

On the other hand, detection and processing in rules is based on the fact that references to parameters are solved right after detection, that is, the tags \texttt{<1>}, \texttt{<2>}, etc. are automatically replaced by the value of the parameters before the processing begins. Positions (attribute \texttt{pos}) specified in instructions such as \texttt{<clip>}, refer to the position of the words inside the chunk.

Also the case policy is automatically applied (see Section \ref{ss:majuscules}) from the pseudolemma of the chunk to the words inside the chunk.


\begin{itemize} \item Input: chunks from the \texttt{in\-ter\-chunk}. \item Output: valid input for the morphological generator of Apertium. \end{itemize}

\paragraph{Data files}

This program has its own specification file, which will have the extension \texttt{.t3x}. Its syntax is based as well on the \texttt{chunker} and the \texttt{in\-ter\-chunk}. \nota{Explicar que no ha de llegir cap fitxer compilat de patrons perquè usa noms i no patrons?}

\paragraph{Pattern matching}

Chunk matching is based on the name of the chunk. Unmatched chunks receive the default processing.

\paragraph{How it works}

The differences with regard to the \texttt{in\-ter\-chunk} module are the following:

\begin{itemize} \item It is not allowed to write chunks (\texttt{<chunk>}) in the output: only lexical units (\texttt{<lu>} or \texttt{<mlu>}) and blanks can be written. \nota{Comprovar aquest ítem perquè era incomplet i l’ha completat mlf} \item New detection attribute \texttt{name} in \texttt{<cat-item>}, which is used in the \texttt{<pattern>} part of rules isolatedly, to force pattern detection basing on its name. \nota{mlf: Què vol dir “de manera aïllada”? Sembla que vulga dir “de tant en tant”. MG: the attribute ’name’ is used in the pattern part of rules? is this correct?} \item Also the attribute \texttt{side} is not used here, as in the \texttt{in\-ter\-chunk}, for the same reason: the bilingual dictionary is not looked up. \nota{MG: però llavors això no és una diferència respecte de \texttt{interchunk} no?} \end{itemize}

\paragraph{Default action}

In this module, the default action is to write the words contained in the chunks, replacing the references with the parameters of the chunk. It will be applied to most chunks, since it is foreseen that this module performs non-default actions only for specific cases requiring some special processing.

Also the case policy is applied by default (see Section \ref{ss:majuscules}).

In any case, blanks outside chunks are copied in the same order as are read, since chunk matching is done individually (this module does not group chunks).

\subsection{Preprocessing of the structural transfer module} \label{ss:preproceso_transfer}

Specification files for the structural transfer modules, also called \emph{transfer rules files}, are pre-processed by the program \textit{apertium-preprocess-transfer}, which calculates the patterns to match rules preconditions, and indexes the rules to speed up its processsing during execution time. This information is saved in a binary file which is read together with the bilingual dictionary and the rules file itself, because the structural transfer and lexical transfer modules are executed together.

\section{De-formatter and re-formatter} \label{se:desformat}

4.1.2 Format processing

This section describes how the de-formatter and re-formatter process the format of the documents. These two modules are created from a set of format specification rules in XML, which are described in Section \ref{ss:reglasformato}.

Apertium can process documents in XML, HTML, RTF and plain text. For all these document types, format is \textit{encapsulated} as explained in the following lines.

Text strings that are identified as part of the format —from now on referred to as \textit{blocks of format} or \textit{superblanks}— are encapsulated between delimiters that depend on the specification of the data flow between modules (which is described in detail in Section~\ref{se:flujodatos}); so, in the flow format (sections \ref{se:noxml1} and \ref{se:noxml2}), \emph{superblanks} are put between brackets ’\texttt{[}’ and ’\texttt{]}’. Each of these encapsulated strings will be treated as it were a blank \texttt{<\textbf{b}/>} (page~\pageref{s3:b}) —that is why they are called \textit{superblanks}— and will be restored in the correct order in the translator’s output.

As has been explained in Section \ref{se:flujodatos}, when the blocks of format are large (as is sometimes the case in HTML with Javascript code fragments, or in RTF with bitmap images), these blocks will be saved as temporary files so that they can be removed from the data flow of the translation.

Sometimes, the format in a document can implicitly indicate the division of the text into sentences (see page \pageref{finfrase} in Section \ref{se:flujodatos}). For example, section or document titles can be a sentence without full stop. If we know that a format mark is indicating this division, we have to take advantage of this information in order to do a better translation. Some examples of format that give us data about the end of a sentence are: two consecutive line breaks in plain text format, a \texttt{</h1>} tag in HTML, etc. The de-formatter generates in such cases a mark of sentence end that is equivalent to a full stop. Format encapsulation method

The types of blocks of format or \emph{superblanks} that can be generated as a result of the format processing are the following:

\begin{itemize} \item \textit{Non-empty blocks of format or superblanks}. They contain exclusively format marks of the source document. In the data flow described in Section~\ref{se:flujodatos} , they begin with a left square bracket ’\texttt{[}’ and end with a right square bracket ’\texttt{]}’. \item \textit{Blocks of format with reference to an external file} or \textit{extensive superblanks}. They encapsulate long format fragments in a way that improves the translator’s performance. In the data flow described in Section~\ref{se:flujodatos}, they begin with the characters ‘


’, then there is the name of the file where the format fragment extracted from the source text is saved, and finally they end with a right square bracket ’\texttt{]}’. \item \textit{Empty blocks of format}. They contain artificial information on text division obtained from the format data. Before the empty block of format, the system places the appropriate artificial punctuation mark. When the original format is restored in the document at the end of the process, the presence of a block of format like this will cause the deletion of the character right before the block in the data flow. \end{itemize}

%% [movido al apéndice] %% Dentro de los bloques de formato, los caracteres ’\texttt{[}’, ’\texttt{]}’, %% @ y ’\verb!\!’ se escapan mediante las secuencias de escape %% ’\verb!\[!’, ’\verb!\]!’, ’\verb!\@!’ y ’\verb!\\!’, respectivamente. Esto %% hay que tenerlo en cuenta para encapsular y desencapsular. En el exterior de %% los bloques de formato es necesario también escapar los corchetes de apertura %% y cierre.

The general criteria applied to the creation of blocks of format are the following:

\label{pg:criteri} \begin{itemize} \item Everything that is considered not to be part of the text to be translated, has to be encapsulated in blocks of format. \item There can not be two or more strictly consecutive non-empty blocks of format. Two consecutive blocks of format must be merged into a single block. \item Empty blocks of format must precede a non-empty block of format or the end of the file. \end{itemize}

Figure~\ref{fg:ejemplopelado} shows an example document the format of which must be processed before translation; the encapsulation corresponds to the flow format not based on XML. Figure~\ref{fg:ejemploencapsulado} displays the result of processing the mentioned document.

\begin{figure}[htbp] \begin{small} \begin{alltt} <html> <head> <title>This is the title</title> <script> <!– ... (an extensive code block) –> </script> </head> <body> <p>This is a paragraph in two lines</p> </body> </html> \end{alltt} \end{small} \caption{Example of HTML document} \label{fg:ejemplopelado} \end{figure}

\begin{figure}[htbp] \begin{small} \begin{alltt} \textbf{[<html> <head> <title>]}This is the title\textbf{.[][@/tmp/temp35345]}This\textbf{[ ]}is a paragraph in two lines\textbf{.[][</p> </body> </html>]} \end{alltt} \end{small} \caption{Example of HTML document where the blocks of format have been encapsulated by the de-formatter}\nota{repeteix coses capítol format -revisar -Gema} \label{fg:ejemploencapsulado} \end{figure}

We would like to emphasize the following from this example: \begin{itemize} \item The system does not generate consecutive blocks of format with content (non-empty). \item Tags like \texttt{</\textbf{title}>} or \texttt{</\textbf{p}>} cause the insertion of an artificial punctuation mark; this insertion is done systematically, even when it is not necessary, because it does not interfere and is efficient. \item Extensive superblanks are literally removed from the translation process. In this case, the temporary file \texttt{temp35345} contains the tags from \texttt{</\textbf{title}>} to \texttt{<\textbf{p}>} \item Simple blanks between words are not encapsulated. But the system does encapsulate multiple blanks (two or more consecutive blanks), tabs, etc. Also line breaks are encapsulated. \end{itemize}

\subsection{Data: format specification rules} \label{ss:reglasformato} This section describes how the de-formatter and re-formatter are generated from a format specification in XML.

Rules for format, like linguistic data, are specified in XML, and they contain regular expressions with \texttt{flex} syntax. The specification is divided in three parts (see its DTD in the Appendix \ref{ss:dtd_formato}):

\begin{itemize} \item \textbf{Configuration options}. Here one specifies the value for the maximum length of a non-extensive superblank, the input and output encodings, whether case must be considered, and the regular expressions for escape characters and space characters.

\item \textbf{Format rules}. Describes the set of tags belonging to a specific format which have to be included in a block of format by the de-formatter. These tags may, optionally, indicate a sentence end, in which case the de-formatter will insert an artificial punctuation mark (followed by an empty block of format, as explained in the previous section). One has to specify the priority of application of the rules, although, when this is not relevant, it is possible to give the same priority to all the rules by assigning them the same value (any number).

Everything that is not specified as format will be left without encapsulation and, therefore, will be considered as translatable text.

\item \textbf{Replacement rules}. Allow to replace special characters in the text. A regular expression will match a set of special characters, and will replace it with the specified characters. For example, in HTML, the characters specified in hexadecimal have to be replaced with the corresponding entity or ASCII character. For example, \texttt{cami\&oacute;n} corresponds to \texttt{camión}. \end{itemize}

Rules are described in more detail next. \begin{itemize} \item Root of the specification file. The attribute \texttt{name} contains the name of the format. \begin{small} \begin{alltt} <?xml version="1.0" encoding="ISO-8859-1"?> <format name="html"> <options> ... </options>

<rules> ... </rules> </format> \end{alltt} \end{small}


It has to include the options and rules, an example of which is presented next:


\item Options. \begin{small} \begin{alltt} <options> <largeblocks size="8192"/> <input encoding="ISO-8859-1"/> <output encoding="ISO-8859-1"/> <escape-chars regexp=’[\verb!\![\verb!\!]^\$\verb!\!\verb!\!]’/> <space-chars regexp=’[ \verb!\!n\verb!\!t\verb!\!r]’/> <case-sensitive value="no"/> </options> \end{alltt} \end{small}


The element \texttt{<largeblocks>} specifies the maximum length of a non-extensive superblank, through the value of the attribute \texttt{size}. The elements \texttt{<input>} and \texttt{<output>} specify the input and output encoding of the text, through the attribute \texttt{encoding}.

The element \texttt{escape-chars} specifies, by means of a regular expression declared in the value of the attribute \texttt{regexp}, which characters must be escaped with a backslash. The element \texttt{<space-chars>} specifies the set of characters that must be considered as blanks.

Finally, the element \texttt{case-sensitive} specifies if case is relevant in the specifications of format attributes in which regular expressions are contained.

\begin{itemize} \item Rules. There are format rules and replacement rules. \begin{small} \begin{alltt} <rules> <format-rule ... > ... </format-rule> ...

<replacement-rule> ... </replacement-rule> ... </rules> \end{alltt} \end{small} The two types are described in the following points.

\item Format rules. The de-formatter will encapsulate in blocks of format the tags indicated by these rules in the field \texttt{regexp}. If they are begin and end tags, and everything delimited by them is format, one has to specify a \texttt{regexp} both for \texttt{begin} and for \texttt{end}: \begin{small} \begin{alltt} <format-rule eos="no" priority="1"> <begin regexp=’"\verb!\!\&lt;!–"’/> <end regexp=’"–\verb!\!\&gt;"’/> </format-rule> \end{alltt} \end{small} Otherwise only one \texttt{begin-end} element is used: \begin{small} \begin{alltt} <format-rule eos="yes" priority="3"> <begin-end regexp=’"\&lt;"[/]?"li"[^\&gt;]*"\&gt;"’/> </format-rule> \end{alltt} \end{small}

Besides, in \texttt{priority} you have to specify a priority to tell the system in which order the format rules must be applied (the absolute value is not relevant, only the order resulting from the values). In “\texttt{eos}” you indicate, with \texttt{yes} or \texttt{no}, whether the block of format that contains the detected pattern must be preceded by an artificial punctuation mark or not.\footnote{In all these cases, the typical entities \texttt{\&lt;} and \texttt{\&gt;} are used to represent the characters \texttt{<} and \texttt{>} respectively.}

\item Replacement rules. Are used to replace special characters in the text. The regular expression in the attribute \texttt{regexp} will recognize \nota{idem: help in translation of "recogerá"} a set of special characters and will replace them with the specified characters in the text to be translated. The correspondence between original and replacement characters is stated in the attributes \texttt{source} and \texttt{target} of the \texttt{replace} elements, which can be multiple: \begin{small} \begin{alltt} <replacement-rule regexp=’"\&amp;"[^;]+;’> <replace source="\&amp;Agrave;" target="À"/> <replace source="\&amp;#192;" target="À"/> <replace source="\&amp;#xC0;" target="À"/> <replace source="\&amp;#xc0;" target="À"/> <replace source="\&amp;Aacute;" target="Á"/> <replace source="\&amp;#193;" target="Á"/> <replace source="\&amp;#xC1;" target="Á"/> <replace source="\&amp;#xc1;" target="Á"/> ... </replacement-rule> \end{alltt} \end{small} \item Regular expressions of \texttt{regexp} attributes. They have the syntax used in \texttt{flex} \cite{lesk75tr}.


% DTD moguda a Apèndix

As example of a format specification, we will give that for HTML. The explanation given in the following paragraphs can be followed looking at Figure \ref{fg:formato-html}.

In the first place, we find the format rule that specifies in a general way all the HTML tags: it considers as HTML tag everything that begins with the sign \textbf{\texttt{<}} and ends with the sign \textbf{\texttt{>}}. This rule has the lowest priority (4) so that the more specific rules are applied preferentially. But before considering a tag in a general way by applying this rule, some of the higher priority rules will be applied. In the case of HTML, the highest priority is for comments \texttt{<!– ... –>}. The marks for beginning and end \texttt{<script> </script>} and \texttt{<style> </style>}, where everything included by them is considered to be format, has priority 2. Priority 3 is for tags that indicate end of sentence (artificial punctuation), which are \texttt{</br>}, \texttt{</hr>}, \texttt{</p>}, etc.

Last of all are the replacement rules, which replace all the codes that begin with \texttt{\&}, as specified in the regular expression. Then, each one of the replacements is defined: \texttt{\&Agrave}, as well as \texttt{\&\#192}, \texttt{\&\#xC0} and \texttt{\&\#xc0} are replaced with \texttt{À}. The remaining special characters are declared in the same way.

\begin{figure}[htbp] \begin{small} \begin{alltt} <?xml version="1.0" encoding="ISO-8859-1"?> <format name="html"> <options> <largeblocks size="8192"/> <input encoding="ISO-8859-1"/> <output encoding="ISO-8859-1"/> <escape-chars regexp=’[\verb!\![\verb!\!]^\$\verb!\!\verb!\!]’/> <space-chars regexp=’[ \verb!\! n\verb!\! t\verb!\! r]’/> <case-sensitive value="no"/> </options>

<rules> <format-rule eos="no" priority="1"> <begin regexp=’"\&lt;!–"’/> <end regexp=’"–\&gt;"’/> </format-rule>

<format-rule eos="no" priority="2"> <begin regexp=’"\&lt;script"[^\&gt;]*"\&gt;"’/> <end regexp=’"\&lt;/script"[^\&gt;]*"\&gt;"’/> </format-rule> <format-rule eos="no" priority="2"> <begin regexp=’"\&lt;style"[^\&gt;]*"\&gt;"’/> <end regexp=’"\&lt;/style"[^\&gt;]*"\&gt;"’/> </format-rule>

<format-rule eos="yes" priority="3"> <begin-end regexp=’"\&lt;"[/]?"br"[^\&gt;]*"\&gt;"’/> </format-rule> <!– Here come more declarations of format-rule eos="yes"–> <!– ... –>

<format-rule eos="no" priority="4"> <begin-end regexp=’"\&lt;"[a-zA-Z][^\&gt;]*"\&gt;"’/> </format-rule>

<replacement-rule regexp=’"\&amp;"[^;]+;’> <replace source="\&amp;Agrave;" target="À"/> <replace source="\&amp;#192;" target="À"/> <replace source="\&amp;#xC0;" target="À"/> <replace source="\&amp;#xc0;" target="À"/> <!– Here come more replace elements –> <!– ... –> </replacement-rule> </rules> </format> \end{alltt} \end{small} \caption{Part of the rules definition for HTML format} \label{fg:formato-html} \end{figure}

\subsection{Generation of de-formatters and re-formatters} \label{se:gendeformat}

To generate the de-formatter and re-formatter for a given format, the XML rules that declare the format are applied a style sheet that carries out the generation. This XSLT transformation produces a \texttt{lex} \cite{lesk75tr} file that, once compiled, is the executable of the de-formatter and the re-formatter for the specified format.

Thanks to the general specification of formats described in this chapter, it has been possible to define specifications for HTML, RTF and plain text. These specifications are in the \texttt{apertium} package, in the respective files \texttt{html-format.xml}, \texttt{rtf-format.xml}, \texttt{txt-format.xml}. In particular, it is quite simple to define de-formatters and re-formatters for any XML format.