3.5.2

### 6Maintaining linguistic data

\nota{Perhaps one could integrate material from Fran Tyers’ howto as found in Apertium Wiki} \section[Description of current data]{Description of linguistic data currently available}

At present, Apertium has linguistic data for three language pairs \nota{MG: This is old, needs UPDATING}: Spanish–Catalan and Spanish–Galician. The files containing the linguistic data are saved in a single directory: \texttt{apertium-es-ca} for the pair Spanish–Catalan and \texttt{apertium-es-gl} for the pair Spanish–Galician. The names of the files in this directory have the following structure:

\begin{itemize}\setlength{\itemsep}{-\parsep} \item \texttt{apertium-PAIR.LANG.dix} : monolingual dictionary for LANG. \item \texttt{apertium-PAIR.LANG1-LANG2.dix} : \texttt{LANG1-LANG2} bilingual dictionary. \item \texttt{apertium-PAIR.trules-LANG1-LANG2.xml} : structural transfer rules for the translation from \texttt{LANG1} to \texttt{LANG2} . \item \texttt{apertium-PAIR.LANG.tsx} : tagger definition file for \texttt{LANG}. \item \texttt{apertium-PAIR.post-LANG.dix} : Post-generation dictionary for \texttt{LANG} (applies when translating into \texttt{LANG}). \item directory \texttt{LANG-tagger-data} : contains data needed for the \texttt{LANG} tagger (corpora, etc.)

\end{itemize}

\texttt{apertium-PAIR} refers to the linguistic combination of the translator. Its two possible values at the moment are \texttt{apertium-es-ca} and \\ \texttt{apertium-es-gl}. According to this structure, the Catalan monolingual dictionary is called \texttt{apertium-es-ca.ca.dix}, the Spanish–Galician bilingual dictionary is called \texttt{apertium-es-gl.es-gl.dix} and the structural transfer rules file for the translation from Catalan into Spanish is called \texttt{apertium-es-ca.trules-ca-es.xml}.

The linguistic data available (by January 2006) for the different language pairs are summarized in the following table. \begin{small} \begin{center} \begin{tabular}{|p{8cm}|p{5cm}|} \hline \multicolumn{2}{|c|}{\textbf{Translator Apertium-es-ca}} \\ \hline Spanish monolingual dictionary & 11.800 entries \\ Catalan monolingual dictionary & 11.800 entries \\ Spanish–Catalan bilingual dictionary & 12.800 entries (correspondences \texttt{es-ca})\\ Structural transfer rules from Spanish into Catalan & 44 rules \\ Structural transfer rules from Catalan into Spanish & 58 rules \\ Spanish post-generation dictionary & 25 entries and 5 paradigms\\ Catalan post-generation dictionary & 16 entries and 57 paradigms\\ \hline \multicolumn{2}{|c|}{\textbf{Translator Apertium-es-gl}} \\ \hline Spanish monolingual dictionary & 9.000 entries \\ Galician monolingual dictionary & 8.600 entries \\ Spanish–Galician bilingual dictionary & 8.500 entries (correspondences \texttt{es-gl})\\ Structural transfer rules from Spanish into Galician & 46 rules \\ Structural transfer rules from Galician into Spanish & 38 rules \\ Spanish post-generation dictionary & 36 entries and 12 paradigms\\ Galician post-generation dictionary & 74 entries and 48 paradigms\\ \hline \end{tabular} \end{center} \end{small}

When extending or adapting Apertium, the most likely operation that will be performed will be to extend its dictionaries. In fact, it will be far more common than adding transfer or post-generation rules.

We describe next the most important things one has to take into account when adding new words to the translator. This information is more general than the data provided in the section describing dictionaries (chapter \ref{ss:diccionarios}), although we give here some practical information that might be very useful to the users who decide to make changes in the translator.

IMPORTANT: Every time a set of modifications is made to any of the dictionaries, the modules have to be recompiled. Type \emph{make} in the directory where the linguistic data are saved (apertium-es-ca, apertium-es-gl or what may be applicable) so that the system generates the new binary files.

If you want to add a new word to Apertium, you need to add three entries in the dictionaries. Suppose you are working with the Spanish-Catalan pair. In this case, you have to add:

\begin{enumerate} \item an entry in the Spanish monolingual dictionary: so that the translator can analyze ("understand") the word when it finds it in a text, and generate it when translating this word into Spanish.

\item an entry in the bilingual dictionary: so that you can tell Apertium how to translate this word from one language to the other.

\item an entry in the Catalan monolingual dictionary: so that the translator can analyze ("understand") the word when it finds it in a text, and generate it when translating this word into Catalan. \end{enumerate}

You will need to go to the directory containing the XML dictionaries (for the Spanish-Catalan pair, this is \texttt{apertium-es-ca}) and open with a text editor or a specialized XML editor the three dictionary files mentioned: \texttt{apertium-es-ca.es.dix}, \texttt{apertium-es-ca.es-ca.dix} and \texttt{apertium-es-ca.ca.dix}. The entries you need to create in these three dictionaries share a common structure. \\

\textbf{Monolingual dictionary (Spanish)}

You may want, for example, to add the Spanish adjective \emph{cósmico}, whose equivalent in Catalan is \emph{còsmic}. The first step is to add this word to the Spanish monolingual dictionary.

You will see that a monolingual dictionary has basically two types of data: \textbf{paradigms} (in the "\texttt{<pardefs>}" section of the dictionary, each paradigm inside a \texttt{<pardef>} element) and \textbf{word entries} (in the main (\texttt{<section>} of the dictionary, each one inside an \texttt{<e>} element). Word entries consist of a lemma (that is, the word as you would find it in a typical paper dictionary) plus grammatical information; paradigms contain the inflection data of all lemmas in the dictionary. You can search a particular word by searching the string \texttt{lm="word"} (\texttt{lm} meaning \emph{lemma}). Bear in mind, however, that the element \texttt{lm} is optional and some other dictionaries may not contain it.

Look at the word entries in the Spanish monolingual dictionary, for example at the entry for the adjective \emph{bonito}. You can find it by searching \texttt{lm="bonito"}:

\begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="bonito"> <\textbf{i}>bonit</\textbf{i}> <\textbf{par} \textsl{n}="absolut/o__adj"/> </\textbf{e}> \end{alltt} \end{small}

To add a word, you will have to create an entry with the same structure. The part between \texttt{<i>} and \texttt{</i>} contains the prefix of the word that is common to all inflected forms, and the element \texttt{<par>} refers to the inflection paradigm of this word. Therefore, this entry means that the adjective \emph{bonito} inflects like the adjective \emph{absoluto} and has the same morphological analysis: the forms \emph{bonit\textbf{o}}, \emph{bonit\textbf{a}}, \emph{bonit\textbf{os}}, \emph{bonit\textbf{as}} are equivalent to the forms \emph{absolut\textbf{o}}, \emph{absolut\textbf{a}}, \emph{absolut\textbf{os}}, \emph{absolut\textbf{as}} and have the morphological analysis: \texttt{adj m sg}, \texttt{adj f sg}, \texttt{adj m pl} and \texttt{adj f pl} respectively.

Now, you have to decide which is the lexical category of the word you want to add: the word \emph{cósmico} is an adjective, like \emph{bonito}. Next, you have to find the appropriate paradigm for this adjective. Is it the same as the one for \emph{bonito} and \emph{absoluto}? ¿Can you say \emph{cósmic\textbf{o}}, \emph{cósmic\textbf{a}}, \emph{cósmic\textbf{os}}, \emph{cósmic\textbf{as}}? The answer is yes, and, with all this information, you can now create the correct entry:

\begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="cósmico"> <\textbf{i}>cósmic</\textbf{i}> <\textbf{par} \textsl{n}="absolut/o__adj"/> </\textbf{e}> \end{alltt} \end{small}

\begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="genoma"> <\textbf{i}>genoma</\textbf{i}> <\textbf{par} \textsl{n}="abismo__n"/> </\textbf{e}> \end{alltt} \end{small}

In exceptional cases you will need to create a new paradigm for a certain word. You can look at the structure of other paradigms and create one accordingly. For a more detailed description of paradigms and word entries in the dictionaries, refer to section \ref{ss:diccionarios}. \\

\textbf{Monolingual dictionary (Catalan)}

Once you have added the word to one monolingual dictionary, you have to do the same to the other monolingual dictionary of the translation pair (in our example, the Catalan monolingual dictionary) using the same structure. The result would be:

\textbf{Monolingual dictionary (Galician)}

In the case you are trying to improve the XML dictionaries for the Spanish-Galician pair, you will need to go to the directory \texttt{apertium-es-gl} and open with a text editor or a specialized XML editor the three dictionary files \texttt{apertium-es-gl.es.dix}, \texttt{apertium-es-gl.es-gl.dix} and \texttt{apertium-es-gl.gl.dix}. In that case, once you have added the new Spanish word \emph{genoma} to the Spanish monolingual dictionary (\texttt{apertium-es-gl.es.dix}), you have to add the equivalent Galician word \emph{xenoma} to the Galician monolingual dictionary (\texttt{apertium-es-gl.gl.dix}), that is:

\begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="xenoma"> <\textbf{i}>xenoma</\textbf{i}> <\textbf{par} \textsl{n}="Xulio__n"/> </\textbf{e}> \end{alltt} \end{small}

\textbf{Bilingual dictionary}

The last step is to add the translation to the bilingual dictionary.

A bilingual dictionary does not usually have paradigms, only lemmas. An entry contains only the lemma in both languages and the first grammatical symbol (the lexical category) of each one. Entries have a left side (\texttt{<l>}) and a right side (\texttt{<r>}), and each language has always to be in the same position: in our system, it has been agreed that Spanish occupies the left side, and Catalan, Galician and Portuguese the right side.

With the addition of the lemma of both words, the system will translate all their inflected forms (the grammatical symbols are copied from the source language word to the target language word). This will only work if the source language word and the target language word are grammatically equivalent, that is, if they share exactly the same grammatical symbols for all of their inflected forms. This is the case with our example; therefore, the entry you have to add to the bilingual dictionary is:

\begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>cósmico<\textbf{s} \textsl{n}="adj"/></\textbf{l}> <\textbf{r}>còsmic<\textbf{s} \textsl{n}="adj"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

This entry will translate all the inflected forms, that is, \texttt{adj m sg}, \texttt{adj f sg}, \texttt{adj m pl} and \texttt{adj f pl}. It works for the translation in both directions: from Spanish to Catalan and from Catalan to Spanish.

In the case of the Spanish-Galician pair, the following bilingual entry in the Spanish-Galician bilingual dictionary (\texttt{apertium-es-gl.es-gl.dix}) will translate all the inflected forms for the equivalent words \emph{genoma}/\emph{xenoma} in both directions:

\begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>genoma<\textbf{s} \textsl{n}="n"/></\textbf{l}> <\textbf{r}>xenoma<\textbf{s} \textsl{n}="n"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

What to do if the word pair is not equivalent grammatically (their grammatical symbols are not exactly the same)? In that case, you need to specify all the grammatical symbols (in the same order as they are specified in the monolingual dictionaries) until you reach the symbol that differs between the source language word and the target language word. For example, the Spanish noun \emph{limón} has masculine gender and its equivalent in Catalan, \emph{llimona}, has feminine gender. The entry in the bilingual dictionary must be as follows:

\begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>limón<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/></\textbf{l}> <\textbf{r}>llimona<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="f"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

"\texttt{LR}" means \emph{left to right} and "\texttt{RL}", \emph{right to left}. Since Spanish is on the left and Catalan on the right, the adjective will be \texttt{GD} only when translating from Spanish to Catalan (\texttt{LR}). For the translation \texttt{RL} you need to create two entries, one for the adjective in feminine and another one for the adjective in masculine.\footnote{You could also group them using a small paradigm}

The same principle applies when it is not possible to determine the number of the target word for the same reasons mentioned above. For example, the Spanish noun \emph{rascacielos} ("skyscraper") is invariable in number, that is, it can be singular as well as plural (\emph{un rascacielos}, \emph{dos rascacielos}). In Catalan, on the other hand, the noun has a different inflection for the singular and for the plural (\emph{un gratacel}, \emph{dos gratacels}). In this case the symbol used is "\texttt{ND}" ("number to be determined") and the entries should be like this:

\begin{small} \begin{alltt} <\textbf{e} \textsl{r}="LR"> <\textbf{p}> <\textbf{l}>rascacielos<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="sp"/></\textbf{l}> <\textbf{r}>gratacel<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="ND"/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e} \textsl{r}="RL"> <\textbf{p}> <\textbf{l}>rascacielos<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="sp"/></\textbf{l}> <\textbf{r}>gratacel<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="pl"/></\textbf{r}> </\textbf{p}> </\textbf{e}> <\textbf{e} \textsl{r}="RL"> <\textbf{p}> <\textbf{l}>rascacielos<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="sp"/></\textbf{l}> <\textbf{r}>gratacel<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/><\textbf{s} \textsl{n}="sg"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

For a more detailed description of this kind of entries, refer to section~\pageref{ss:bil}.

In the previous example we have already seen the use of direction restrictions for entries with undetermined gender or number (\texttt{GD} or \texttt{ND}). These restrictions can also be used in other cases.

It is important to note that the current version of Apertium can give only a single equivalent for each source-language lexical form \nota{NEEDS UPDATING, reference to lextor} (a lexical form is the lemma plus its grammatical information), that is, no word-sense disambiguation is performed.\footnote{The system performs only part-of-speech disambiguation for homograph words, that is, for ambiguous words that can be analyzed as more than one lexical form, like \emph{vino} in Spanish, that can mean both "wine" and "he/she came". This type of disambiguation is performed by the tagger.} When a lexical form can be translated in two or more different ways, one has to be chosen (the most general, the most frequent, etc.). You can tell Apertium that a certain word has to be analyzed ("understood") but not generated, as it is not the translation of any word in the other language.

Let’s see this with an example. The Spanish noun \emph{muñeca} can be translated in two different ways in Catalan depending on its meaning: \emph{canell} ("wrist") or \emph{nina} ("doll"). The context decides which translation is the correct one, but in its present state Apertium can not make such a decision .\footnote{See Section \ref{multi} on multiword units for ways to circumvent this problem.} Therefore, you have to decide which word you want as an equivalent when translating from Spanish to Catalan. From Catalan to Spanish, both words can be translated as \emph{muñeca} without any problem. You have to specify all these circumstances in the dictionary entries using direction restrictions (\texttt{LR} meaning "left to right", that is, \texttt{es}–\texttt{ca}, and \texttt{RL} meaning "right to left", that is, \texttt{ca}–\texttt{es}). If you decide to translate \emph{muñeca} as \emph{canell} in all cases, the entries in the bilingual dictionary shall be:

\begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>muñeca<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="f"/></\textbf{l}> <\textbf{r}>canell<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/></\textbf{r}> </\textbf{p}> </\textbf{e}>

<\textbf{e} \textsl{r}="RL"> <\textbf{p}> <\textbf{l}>muñeca<\textbf{s} \textsl{n}="n"/></\textbf{l}> <\textbf{r}>nina<\textbf{s} \textsl{n}="n"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

This means that translation directions will be: \begin{small} \begin{alltt} muñeca –> canell muñeca <– canell muñeca <– nina \end{alltt} \end{small}

(Note that that there is also a gender change in the case of \emph{muñeca} (feminine) and \emph{canell} (masculine)).

It should be emphasized that a lemma can not have two translations in the target language, because the system would give an error when translating that lemma (see Section \ref{errores} "Detecting errors" to see how to find and correct these and other types of errors). When a word can be translated in two different ways in the target language in all contexts, you need to choose one as the translation equivalent and leave the other one as a lemma that can be analyzed but not generated, using direction restrictions like in the previous example. For example, the Catalan lemmas \emph{mot} and \emph{paraula} can be both translated into Spanish as \emph{palabra} ("word") and the entries in the bilingual dictionary should look like this:

\begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>palabra<\textbf{s} \textsl{n}="n"/></\textbf{l}> <\textbf{r}>paraula<\textbf{s} \textsl{n}="n"/></\textbf{r}> </\textbf{p}> </\textbf{e}>

<\textbf{e} \textsl{r}="RL"> <\textbf{p}> <\textbf{l}>palabra<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="f"/></\textbf{l}> <\textbf{r}>mot<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

Therefore, for this lemmas the translation directions will be: \begin{small} \begin{alltt} palabra –> paraula palabra <– paraula palabra <– mot \end{alltt} \end{small}

One may have to specify restrictions regarding translation direction also in monolingual dictionaries. For example, both Spanish forms \emph{cantaran} and \emph{cantasen} should be analyzed as lemma \emph{cantar}, verb, subjunctive imperfect, 3rd person plural, but when generating Spanish text, one has to decide which one will be generated. Monolingual dictionaries are read in two directions depending on its purpose: for the analysis, the reading direction is left to right; for the generation, right to left. Therefore, a word that must be analyzed but not generated must have the restriction \texttt{LR}, and a word that must be generated but not analyzed must have the restriction \texttt{RL}.

The case of \emph{cantaran} or \emph{cantasen} must have already been taken care of in inflection paradigms and it is unlikely to be a problem for most people extending a dictionary. In some other cases it can be necessary to introduce a restriction in the word entries of monolingual dictionaries.

It is possible to create entries consisting of two or more words, if these words are considered to build a single "translation unit". These multiword units can also be useful when it comes to select the correct equivalent for a word inside a fixed expression. For example, the Spanish word \emph{dirección} may be translated into two Catalan words: \emph{direcció} ("direction, management, directorate, steering", etc.) and \emph{adreça} ("address"); including, for example, frequent multiword units such as \emph{dirección general} $$\to$$ \emph{direcció general} ("general directorate") and \emph{dirección postal} $$\to$$ \emph{adreça postal} ("postal address") may help get improved translations in some situations.

Multiword units can be classified basically into two categories: multiwords with inner inflection and multiwords without inner inflection.

\subsubsection{Multiwords without inner inflection}

They are just like the normal one-word entries, with the only difference that you need to insert the element \texttt{<b>} (which represents a blank) between the individual words that make up the unit. Therefore, if you want to add, for example, the Spanish multiword \emph{hoy en día} ("nowadays"), whose equivalent in Catalan is \emph{avui dia}, the entries you need to add to the different dictionaries are:

\begin{itemize}

\item Spanish monolingual dictionary: \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="hoy en día"> <\textbf{i}>hoy<\textbf{b}/>en<\textbf{b}/>día</\textbf{i}> <\textbf{par} \textsl{n}="ahora__adv"/> </\textbf{e}> \end{alltt} \end{small}

\item Catalan monolingual dictionary: \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="avui dia"> <\textbf{i}>avui<\textbf{b}/>dia</\textbf{i}> <\textbf{par} \textsl{n}="ahir__adv"/> </\textbf{e}> \end{alltt} \end{small}

\item Spanish-Catalan bilingual dictionary: \begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>hoy<\textbf{b}/>en<\textbf{b}/>día<\textbf{s} \textsl{n}="adv"/></\textbf{l}> <\textbf{r}>avui<\textbf{b}/>dia<\textbf{s} \textsl{n}="adv"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

\end{itemize}

For Spanish-Galician pair, if you want to add, for example, the Spanish multiword \emph{manga por hombro} ("disarranged"), whose equivalent in Galician is \emph{sen xeito nin modo}, the entries you need to add are:

\begin{itemize}

\item Spanish monolingual dictionary: \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="manga por hombro"> <\textbf{i}>manga<\textbf{b}/>por<\textbf{b}/>hombro</\textbf{i}> <\textbf{par} \textsl{n}="ahora__adv"/> </\textbf{e}> \end{alltt} \end{small}

\item Galician monolingual dictionary: \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="sen xeito nin modo"> <\textbf{i}>sen<\textbf{b}/>xeito<\textbf{b}/>nin<\textbf{b}/>modo</\textbf{i}> <\textbf{par} \textsl{n}="Deo_gratias__adv"/> </\textbf{e}> \end{alltt} \end{small}

\item Spanish-Galician bilingual dictionary: \begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>manga<\textbf{b}/>por<\textbf{b}/>hombro<\textbf{s} \textsl{n}="adv"/></\textbf{l}> <\textbf{r}>sen<\textbf{b}/>xeito<\textbf{b}/>nin<\textbf{b}/>modo<\textbf{s} \textsl{n}="adv"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

\end{itemize}

The paradigms of the previous examples, as adverbs do not inflect, contain only the grammatical symbol of the lexical form, as you see in this example:

\begin{small} \begin{alltt} <\textbf{pardef} \textsl{n}="ahora__adv"> <\textbf{e}> <\textbf{p}> <\textbf{l}/> <\textbf{r}><\textbf{s} \textsl{n}="adv"/></\textbf{r}> </\textbf{p}> </\textbf{e}> </\textbf{pardef}> \end{alltt} \end{small}

Paradigms are build like a lexical entry. We have seen so far lexical entries where the common part of the lemma is put between \texttt{<i>} \texttt{</i>}:

\begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="cósmico"> <\textbf{i}>cósmic</\textbf{i}> <\textbf{par} \textsl{n}="absolut/o__adj"/> </\textbf{e}> \end{alltt} \end{small}

But you can also express the same with a pair of strings: a left string \texttt{<l>} and a right string \texttt{<r>} inside a \texttt{<p>} element:

\begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="cósmico"> <\textbf{p}> <\textbf{l}>cósmic</\textbf{l}> <\textbf{r}>cósmic</\textbf{r}> </\textbf{p}> <\textbf{par} \textsl{n}="absolut/o__adj"/> </\textbf{e}> \end{alltt} \end{small}

These two entries are equivalent. The use of the \texttt{<i>} element helps get more simple and compact entries, and you can use it when the left side and the right side of the string pair are identical. As has been explained before, monolingual dictionaries are read \texttt{LR} for the analysis of a text and \texttt{RL} for the generation. Therefore, when there is some difference between the analysed string and the generated string (not very usual) the entry can not be written using the \texttt{<i>} element. This is what happens in paradigms, where the left and right strings are never identical, since the right side must contain the grammatical symbols that will go through all the modules of the system.

\subsubsection{Multiwords with inner inflection}

They consist of a word that can inflect (typically a verb) followed by one or more invariable words. For these entries you need to specify the inflection paradigm just after the word that inflects. The invariable part must be marked with the element \texttt{<g>} (for \emph{group}) in the right side. The blanks between words are indicated, like in the previous case, with the element \texttt{<b>}. Look at the following example for the Spanish multiword \emph{echar de menos} (to miss), translated into Catalan as \emph{trobar a faltar}:

\begin{itemize}

\item Spanish monolingual dictionary: \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="echar de menos"> <\textbf{i}>ech</\textbf{i}> <\textbf{par} \textsl{n}="aspir/ar__vblex"/> <\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}

\item Catalan monolingual dictionary: \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="trobar a faltar"> <\textbf{i}>trob</\textbf{i}> <\textbf{par} \textsl{n}="abander/ar__vblex"/> <\textbf{p}> <\textbf{l}><\textbf{b}/>a<\textbf{b}/>faltar</\textbf{l}> <\textbf{r}><\textbf{g}><\textbf{b}/>a<\textbf{b}/>faltar</\textbf{g}></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

\item Spanish-Catalan bilingual dictionary: \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}

\end{itemize}

Note that the grammatical symbol is appended at the end, after the group marked with the \texttt{<g>}.

It can be the case that a lemma is a multiword of this kind in one language and a single word in the other language. In that case, in the bilingual dictionary, the multiword will contain the \texttt{<g>} element and the single word will not. In the monolingual dictionaries, each entry will be created according to its type. Look at the following example for the Spanish multiword \emph{darse cuenta} (to realize), translated into Catalan as the verb \emph{adonar-se}:\footnote{The verb \emph{adonar-se} is considered a simple word, since the incorporation of enclitic pronouns (such as "-se") is treated inside the inflection paradigms of verbs (for all the Romance languages of \emph{Apertium}); therefore, it is not necessary to specify them in lexical entries. The correct placement of clitic pronouns is one of the main reasons for using the \texttt{<g>}... \texttt{</g>} labels around the invariable part of multi-word verbs.}

\begin{itemize}

\item Spanish monolingual dictionary: \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="darse cuenta"> <\textbf{i}>d</\textbf{i}> <\textbf{par} \textsl{n}="d/ar__vblex"/> <\textbf{p}> <\textbf{l}><\textbf{b}/>cuenta</\textbf{l}> <\textbf{r}><\textbf{g}><\textbf{b}/>cuenta</\textbf{g}></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

\item Catalan monolingual dictionary: \begin{small} \begin{alltt} <\textbf{e} \textsl{lm}="adonar-se"> <\textbf{i}>adon</\textbf{i}> <\textbf{par} \textsl{n}="abander/ar__vblex"/> </\textbf{e}> \end{alltt} \end{small}

\item Spanish-Catalan bilingual dictionary: \begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>dar<\textbf{g}><\textbf{b}/>cuenta</\textbf{g}><\textbf{s} \textsl{n}="vblex"/></\textbf{l}> <\textbf{r}>adonar<\textbf{s} \textsl{n}="vblex"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small}

\end{itemize}

The same principles and actions described for basic entries (gender and number change, direction restrictions, etc.) apply to all kinds of multiwords. For a more detailed description of multiword units, refer to section~\ref{ss:multipalabras}.

\subsection{Consider contributing your improved lexical data}

If you have successfully added general-purpose lexical data to any of the Apertium language pairs, please consider contributing it to the project so that we can offer a better toolbox to the community. You can e-mail your data (in three XML files, one for each monolingual dictionary and another one for the bilingual dictionary) to the following addresses: \\

\begin{tabular}{ll} Spanish-Catalan data & Mireia Ginestí: \texttt{mginesti@dlsi.ua.es}\\ Spanish-Portuguese data & Carme Armentano: \texttt{carmentano@dlsi.ua.es}\footnote{The group at the Universitat d’Alacant has also developed data for this language pair outside the present project.}\\ Spanish-Galician data & Xavier Gómez-Guinovart: \texttt{xgg@uvigo.es}\\\\

\end{tabular}

If you believe you are going to contribute more heavily to the project, you can join the development team through www.sourceforge.net. If you do not have a Sourceforge account, please create one; then write to Mikel L. Forcada (\texttt{mlf@ua.es}) or Sergio Ortiz (\texttt{sortiz@dlsi.ua.es}), or to Xavier Gómez Guinovart if you are interested in the Spanish-Galician language pair, explaining briefly your motivations and background to join the project. The usual way to contribute is to use CVS; as a project member, you will be able to commit your changes to dictionaries directly.

The addition of simple lexical contributions will soon be made simpler by means of web forms in \url{http://xixona.dlsi.ua.es/prototype/webform/}, so that contributors do not have to deal directly with XML.

You should be aware that the data you contribute to the project, once added, will be freely distributed under the current license (GNU General Public License or Creative Commons 2.5 attribution-sharealike-noncommercial, as indicated). Make sure the data you contribute is not affected by any kind of license which may be incompatible with the licenses used in this project. No kind of agreement or contract is created between you and the developers. If you have any doubt, or you plan to make a massive contribution, contact Mikel L. Forcada.

The content in this chapter partially repeats information already presented in the chapter describing the structural transfer module (Section \ref{ss:transfer}), although rules are described here in a more general and practical way, aimed at those who wish a first approach to them.

Structural transfer rules carry out transformations to the analysed and disambiguated text, which are needed because of grammatical, syntactical and lexical divergences between the two languages involved (gender and number changes to ensure agreement in the target language, word reorderings, changes in prepositions, etc.). The rules detect patterns (sequences) of source text lexical forms and apply to them the corresponding transformations. The module detects the patterns in a left-to-right, longest-match way; for example, the phrase \emph{the big cat} will be detected and processed by the rule for \emph{determiner}–\emph{adjective}–\emph{noun} and not by the rule for \emph{determiner}–\emph{adjective}, since the first pattern is longer. If two patterns have the same length, the rule that applies is the one defined in the first place.

The structural transfer module (generated from the structural transfer rules file) calls the lexical transfer module (generated from the bilingual dictionary) all through the process to determine the target language equivalents of the source language lexical forms.

The structural transfer rules are contained in a XML file, one for each translation direction (for example, for the translation from Spanish to Catalan, the file is \texttt{apertium-es-ca.trules-es-ca.xml}). You need to edit this file if you want to add or change transfer rules.

Rules have a \textbf{pattern} and an \textbf{action} part. The pattern specifies which sequences of lexical forms have to be detected and processed. The action describes the verifications and transformations that need to be done on its constituents. Usual transformation operations (such as gender and number agreement) are defined inside a macroinstruction which is called inside the rule. At the end of the action part of the rule, the resulting lexical forms in the target language are sent out so that they are processed by the next modules in the translation system.

A transfer rules file contains four sections with definitions of elements used in the rules, and a fifth section where the actual rules are defined. The sections are the following:

\begin{itemize}

\item \texttt{<section-def-cats>}: This section contains the definition of the categories which are to be used in the rule patterns (that is, the type of lexical forms that will be detected by a certain rule). For the rule presented below, the categories \texttt{det} and \texttt{nom} (determiner and noun) need to be defined here. Categories are defined specifying the grammatical symbols that the lexical forms have. An asterisk indicates that one or more grammatical symbols follow the ones specified. The following is the definition of the category \texttt{det}, which groups determiners and predeterminers\footnote{such as in Spanish \emph{todo}, \emph{toda}, \emph{todos}, \emph{todas}} in the same category since they play the same role for transfer purposes:

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

It is also possible to define as a category a certain lemma, like the following for the preposition \texttt{en}:

\begin{small} \begin{alltt} <\textbf{def-cat} \textsl{n}="en"> <\textbf{cat-item} \textsl{lemma}="en" \textsl{tags}="pr"/> </\textbf{def-cat}> \end{alltt} \end{small}

\item \texttt{<section-def-attrs>}: This section contains the definition of the attributes that will be used inside of the rules, in the action part. You need attributes for all the categories defined in the previous section, if they are to be used in the action part of the rule (to make verifications on them or to send them out at the end of the rule), as well as for other attributes needed in the rule (such as gender or number). Attributes have to be defined using their corresponding grammatical symbols and can not have asterisks; its name must be unique. The following are the definitions for the attributes \texttt{a\_det} (for determiners) and \texttt{gen} (gender):

\begin{small} \begin{alltt} <\textbf{def-attr} \textsl{n}="a_det"> <\textbf{attr-item} \textsl{tags}="det.def"/> <\textbf{attr-item} \textsl{tags}="det.ind"/> <\textbf{attr-item} \textsl{tags}="det.dem"/> <\textbf{attr-item} \textsl{tags}="det.pos"/> <\textbf{attr-item} \textsl{tags}="predet"/> </\textbf{def-attr}>

<\textbf{def-attr} \textsl{n}="gen"> <\textbf{attr-item} \textsl{tags}="m"/> <\textbf{attr-item} \textsl{tags}="f"/> <\textbf{attr-item} \textsl{tags}="mf"/> <\textbf{attr-item} \textsl{tags}="nt"/> <\textbf{attr-item} \textsl{tags}="GD"/> </\textbf{def-attr}>

\end{alltt} \end{small}

\item \texttt{<section-def-vars>}: This section contains the definition of the variables used in the rules.

\begin{small} \begin{alltt} <\textbf{def-var} \textsl{n}="interrogativa"/> \end{alltt} \end{small}

\item \texttt{<section-def-macros>}: Here the macroinstructions are defined, which contain sequences of code that are frequently used in the rules; this way, linguists do not need to write the same actions repeatedly. There are, for example, macroinstructions for gender and number agreement operations.

\item \texttt{<section-def-rules>}: This is the section where the structural transfer rules are written.

\end{itemize}

The following is an example of a rule which detects the sequence \emph{determiner–noun}:

\begin{small} \begin{alltt} <\textbf{rule}> <\textbf{pattern}> <\textbf{pattern-item} \textsl{n}="det"/> <\textbf{pattern-item} \textsl{n}="nom"/> <\textbf{/pattern}> <\textbf{action}> <\textbf{call-macro} \textsl{n}="f_concord2"> <\textbf{with-param} \textsl{pos}="2"/> <\textbf{with-param} \textsl{pos}="1"/> </\textbf{call-macro}> <\textbf{out}> <\textbf{lu}> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="whole"/> </\textbf{lu}> <\textbf{b} \textsl{pos}="1"/> <\textbf{lu}> <\textbf{clip} \textsl{pos}="2" \textsl{side}="tl" \textsl{part}="whole"/> </\textbf{lu}> </\textbf{out}> </\textbf{action}> </\textbf{rule}> \end{alltt} \end{small}

Part of the action performed on this pattern is specified inside the macroinstruction \texttt{f\_concord2}, which is defined in the \texttt{<section-def-macros>}. It performs gender and number agreement operations: if there is a gender or number change between the source language and the target language (in the noun), the determiner changes its gender or number accordingly; furthermore, if gender or number are undetermined (\texttt{GD} or \texttt{ND}\footnote{See pages \pageref{pg:GD} or \pageref{GDND}}), the noun receives the correct gender or number values from the preceding determiner. In the Apertium es–ca, es–gl and es–pt systems, there are agreement macroinstructions defined for one, two, three or four lexical units (\texttt{f\_concord1}, \texttt{f\_concord2}, \texttt{f\_concord3}, \texttt{f\_concord4}). When calling the macroinstructions in a rule, it must be specified which is the main lexical unit (the one which most heavily determines the gender or number of the other lexical units) and which other lexical units of the pattern have to be included in the agreement operations, in order of importance. This is done with the \texttt{<with-param pos=""/>} element. In the presented rule, the main lexical unit is the noun (position "2" in the pattern) and the second one is the determiner (positions "1" in the pattern).

After the pertinent actions, the resulting lexical forms are sent out, inside the \texttt{<out>} element. Each lexical unit is defined with a \texttt{<clip>}. Its attributes mean the following:

\begin{itemize}

\item [-]\texttt{pos}: refers to the position of the lexical form in the pattern; \texttt{1} is the first lexical form (the determiner) and \texttt{2} the second one (the noun).

\item [-]\texttt{side}: indicates if the lexical form is in the source language (\texttt{sl}) or in the target language (\texttt{tl}). Of course, words are sent out always in the target language; source language lexical forms may be needed inside of a rule, when testing its attributes or characteristics.

\item [-]\texttt{part}: indicates which part of the lexical form is referred to in the \texttt{clip}. You can use some predefined values:

\begin{itemize}

\item [-]\texttt{whole}: the whole lexical form (lemma and grammatical symbols). Used only when sending out the lexical unit (inside an \texttt{<out>} element).

\item [-]\texttt{lem}: the lemma of the lexical unit

\item [-]\texttt{lemh}: the head of the lemma of a multiword with inner inflection (see Section \ref{multi} in this chapter, or Section~\ref{ss:multipalabras} if you wish a more detailed description)

\item [-]\texttt{lemq}: the queue of a lemma of a multiword with inner inflection

\end{itemize}

Apart from these predefined values, you can use any of the attributes defined in \texttt{<section-def-attrs>} (for example \texttt{gen} or \texttt{a\_det}).

The values \texttt{lemh} and \texttt{lemq} are used when sending out multiwords with inner inflection in order to place the head and the queue of the lemma in the right position, since the previous module moved the queue just after the lemma head for various reasons. In practice, in our system, this means that you must use these values instead of \texttt{whole} when sending out verbs. This is because, in our dictionaries, multiwords with inner inflection are always verbs \nota{NEEDS UPDATING}and, if you use the value \texttt{whole} when sending them out, the multiword would not be well formed (the head and the queue of the lemma would not have the correct position and the multiword could not be generated by the generator).

\end{itemize}

Therefore, a rule that has a verb in its pattern must send the lexical forms like in the following two examples:

\label{regla_verbo1} \begin{small} \begin{alltt} <\textbf{rule}> <\textbf{pattern}> <\textbf{pattern-item} \textsl{n}="verb"/> <\textbf{/pattern}> <\textbf{action}> <\textbf{out}> <\textbf{lu}> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="lemh"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="a_verb"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="temps"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="persona"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="gen"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="nbr"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="lemq"/> </\textbf{lu}> </\textbf{out}> </\textbf{action}> </\textbf{rule}> \end{alltt} \end{small}

\label{regla_verbo2} \begin{small} \begin{alltt} <\textbf{rule}> <\textbf{pattern}> <\textbf{pattern-item} \textsl{n}="verb"/> <\textbf{pattern-item} \textsl{n}="prnenc"/> <\textbf{/pattern}> <\textbf{action}> <\textbf{out}> <\textbf{mlu}> <\textbf{lu}> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="lemh"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="a_verb"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="temps"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="persona"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="nbr"/> </\textbf{lu}> <\textbf{lu}> <\textbf{clip} \textsl{pos}="2" \textsl{side}="tl" \textsl{part}="lem"/> <\textbf{clip} \textsl{pos}="2" \textsl{side}="tl" \textsl{part}="a_prnenc"/> <\textbf{clip} \textsl{pos}="2" \textsl{side}="tl" \textsl{part}="persona"/> <\textbf{clip} \textsl{pos}="2" \textsl{side}="tl" \textsl{part}="gen"/> <\textbf{clip} \textsl{pos}="2" \textsl{side}="tl" \textsl{part}="nbr"/> <\textbf{clip} \textsl{pos}="1" \textsl{side}="tl" \textsl{part}="lemq"/> </\textbf{lu}> </\textbf{mlu}> </\textbf{out}> </\textbf{action}> </\textbf{rule}> \end{alltt} \end{small}

The first rule detects a verb and places the queue in the correct place, after all the grammatical symbols. The lexical unit is sent specifying the attributes separately: lemma head, lexical category (verb), tense, person, gender (for the participles), number and lemma queue.

The second rule detects a verb followed by an enclitic pronoun and sends the two lexical forms specifying also the attributes separately; the first lexical unit consists of: lemma head, lexical category (verb), tense, person and number; the second lexical unit consists of: lemma, lexical category (enclitic pronoun), person, gender, number and lemma queue (of the first lexical form). This way, the queue of the lemma is placed after the enclitic pronoun. The two lexical units (verb and enclitic pronoun) are sent inside a \texttt{<mlu>} element, since they have to reach the morphological generator as a multilexical unit (multiword).

Taking into account what we have explained here, if you want to \textbf{add a new transfer rule} you have to follow these steps:

\begin{enumerate}

\item Specify which pattern you want to detect. Bear in mind that words are processed only once by a rule, and that rules are applied left to right and choosing the longest match. For example, imagine you have in your transfer rules file only two rules, one for the pattern \emph{determiner–noun} and one for the pattern \emph{noun–adjective}. The Spanish phrase \emph{el valle verde} ("the green valley") would be detected and processed by the first one, not by the second. You will need to add a rule for the pattern \emph{determiner - noun - adjective} if you wish that the three lexical units are processed in the same pattern.

\item Describe the operations you want to perform on the pattern. In the Apertium \texttt{es-ca}, \texttt{es-gl} and \texttt{es-pt} systems, simple agreement operations (gender and number agreement) are easy to perform in a rule by means of a macroinstruction. To perform other operations, you will need to use more complicated elements; for a more detailed description of the language used to create rules, refer to the section \ref{formatotransfer}.

\item Send the lexical units of the pattern in the target language inside an \texttt{<out>} element. Each lexical unit must be included in a \texttt{<lu>} element. If two or more lexical units must be generated as a multilexical unit (only for enclitic pronouns in the present language pairs) , they must be grouped inside a \texttt{<mlu>} element.

All the words that are detected by a rule (that are part of a pattern) must be sent out at the end of the rule so that the next module (the generator) receives them. If a lexical unit is detected by a pattern and is not included in the \texttt{<out>} element, it will not be generated.

\end{enumerate}

\section[Adding data for the part-of-speech tagger]{Adding data for the lexical categorial disambiguator (part-of-speech tagger)}

The lexical categorial disambiguator takes the linguistic information needed to disambiguate a text basically from two sources: a tagset definition file and corpora. The tagset definition file is contained in the linguistic data directory and its name has the structure \texttt{apertium-PAIR.LANG.tsx}, whereas corpora information is contained in the \texttt{LANG-tagger-data} directory included in the previous directory.

The \emph{tagset definition file} contains the definition of the coarse tags (or categories) used by the tagger when being trained and when disambiguating a text, as well as tag co-occurrence restrictions that help obtain better tag probabilities. In Section \ref{ss:tagger} you can find a detailed description of its characteristics.

The \emph{corpora} that need to be in the \texttt{LANG-tagger-data} directory are different depending on whether the tagger is trained in a supervised way (with manually disambiguated text) or unsupervised (without manually disambiguated text):

\begin{itemize}

\item to train the tagger in a supervised way you need the files (examples from es-tagger-data): \texttt{es.tagged.txt}, \texttt{es.untagged}, \texttt{es.tagged}, \texttt{es.dic}.

\item to train the tagger in an unsupervised way you need the files (examples from es-tagger-data): \texttt{es.crp.txt}, \texttt{es.crp}, \texttt{es.dic}

\end{itemize}

These files have the following characteristics:

\begin{itemize}

\item \texttt{es.tagged.txt}: A Spanish corpus in plain text format. \item \texttt{es.untagged}: The corpus \texttt{es.tagged.txt} morphologically analysed, which means, processed by the de-formatter and the morphological analyser (automatically generated corpus). \item \texttt{es.tagged}: The preceding corpus manually disambiguated. \item \texttt{es.crp.txt}: A large corpus (hundreds of thousands of words) used when training the tagger in an unsupervised way with Baum-Welch reestimation. \item \texttt{es.crp}: The preceding corpus processed consecutively by the de-formatter and the morphological analyser (automatically generated corpus). \item \texttt{es.dic}: File created from the Spanish monolingual dictionary \texttt{*.es.dix}, by means of the \texttt{lt-expand} and \texttt{aper\-tium\–fil\-ter\–am\-biguity} tools, which expand the dictionary and filter the ambiguity classes, so that the file contains all the forms identified as different ambiguity classes by the tagger defined with \texttt{*.es.tsx}; that is, which lexical categories can be homographs (automatically generated corpus). \end{itemize}

When downloading Apertium from Sourceforge (\url{http://apertium.sourceforge.net/}), if the tagger has been trained in a supervised way, it is probable that you get the files needed for this kind of training, \texttt{es.tagged} and \texttt{es.tagged.txt} (for Spanish). The other required files are automatically generated when running the training. If the tagger has been trained in an unsupervised way, you will not get any corpus in the download since the files required for this kind of training are huge. If you wish to train the tagger with this method, you will need to collect a large corpus and name it \texttt{es.crp.txt}. The other required files are automatically generated when running the training.

Anyway, the Apertium translator comes with all the data required for a good performance of the tagger. You don’t need to train the tagger in order to use Apertium. A retraining might be required in the case that you have made really extensive changes to the dictionaries or you have modified the tagset definition file.

Therefore, the tagger data can be modified in two ways:

\begin{enumerate}

\item Change the tagset definition file. You can add, change or delete the coarse tags used by the tagger, if you think that a new category could be useful for the disambiguation or that a certain category should be modified to obtain better results. You can also add restrictions (for example, you can forbid the sequence determiner–determiner if this is an impossible combination in a given language and can help in the disambiguation of certain homograph words).

\item Modify the corpora used to train the tagger. You can modify the manually disambiguated text (\texttt{es.tagged} for Spanish) if you think that certain tags have been wrongly selected. You can also add sentences to this text (and to \texttt{es.tagged.txt}, used to automatically generate the corpus \texttt{es.untagged}) in order to add information to the tagger, since it is possible that certain combinations are incorrectly disambiguated because the tagger has not found them in the training corpora.

\end{enumerate}

There are two commands to run the training:

\begin{itemize}

\item to train in a supervised way, type, in the directory containing the linguistic data (example for \emph{es}–\emph{ca}): \texttt{make -f es-ca-supervised.make}

\item to train in an unsupervised way, type, in the directory containing the linguistic data (example for \emph{es}–\emph{ca}): \texttt{make -f es-ca-unsupervised.make}

\end{itemize}

In both cases, planned files will be automatically generated.

\section{Detecting errors} \label{errores}

It is easy to make errors when adding new words or transfer rules to the Apertium system.

On the one hand, it is possible that, when compiling the new files, the system displays an error message. In this case, this is a formal error (a missing XML tag, a tag that is not allowed in a certain context, etc.). You just have to go to the line number indicated by the error message, correct the error and compile again. On the other hand, there are other types of errors not detected when compiling, but which can make the system mistranslate a word or give an incomprehensible text string. These are linguistic errors, which can be detected and corrected with the tips given in this chapter. The following information is for Linux users, since Apertium works for the moment only in this operating system.\footnote{There are in \url{https://apertium.org} experimental packages for Windows with fixed linguistic data (non-modifiable binary files).}

When the system encounters a problem to translate any word of a source language text, in the default mode the system outputs the problematic word together with a symbol that indicates that an error has occurred. The meaning of the different symbols is the following:

\begin{itemize}

\item ’\verb!@!’: The problem is in the lexical transfer module, which can not translate the lexical form (the bilingual dictionary does not contain it)

\item ’\verb!#!’: The problem has occurred in the generator, which can not generate the surface form from the input lexical form (the morphological dictionary does not contain it in the generation direction)

\item ’\verb!/!’: This symbol separates two or more surface forms delivered by the generator. The problem, therefore, is in the target language monolingual dictionary, which has, in the generation direction, two surface forms for a single lexical form, when it should have only one.

\end{itemize}

The generation module has three modes, which enable us to decide how errors will be displayed in the final output. The three possible parameters are:

\begin{itemize}

\item -n : error symbols and the unknown-word symbol will NOT be displayed, and neither will any grammatical symbols

\item -g : error symbols and the unknown-word symbol will be displayed (default mode)

\item -d : error symbols and the unknown-word symbol will be displayed, as well as the grammatical symbols of the lexical forms producing the error.

\end{itemize}

The preferable mode depends on the type of user and on the translation purpose. The first option is the most suitable when the user does not want that external signs interfere in the reading of the translation. The second option is useful when the user wants the system to show where there has been a problem in the translation (errors or unknown words) in order to be able to post-edit it easily. The third option is ideal for linguistic developers of Apertium, since it displays all the linguistic information of the forms that produced an error.

Taking advantage of the error symbols output by the system, it is possible to carry out a thorough test of the dictionaries of a certain language pair. This will enable you to detect and correct all its errors. To learn how to do it, see Section \ref{integridad}.

\subsection{Output of the different Apertium modules}

Sometimes it is difficult to find the origin of an error. In such cases, it is useful to see the output of each of the modules. As all the data processed by the system, from the original text to the translated text, circulate between the eight modules of the system in text format, it is possible to stop the text stream at any point to know what is the input or the output of a certain module.

Using a pipeline structure and the \texttt{echo} or \texttt{cat} commands, you can send a text through one or more modules to analyse their output and detect the origin of the error. We describe next how to do it. You have to move to the directory where the linguistic data are saved and type the described commands.

\subsubsection{The morphological analyser output}

To know how a word is analyzed by the translator, type the following in the terminal (example for the Catalan word \emph{sabates}):

\begin{small} \begin{alltt} echo "sabates" | apertium-destxt | lt-proc ca-es.automorf.bin \end{alltt} \end{small}

You can replace \texttt{ca-es} with the translation direction you want to test.

The output in Apertium should be: \begin{small} \begin{alltt} ^sabates/sabata<n><f><pl>\$^./.<sent>\$[][] \end{alltt} \end{small}

The string structure is \verb!^!\texttt{word/lemma<}\textsl{morphological analysis}\texttt{>}\verb!$!. The \texttt{<sent>} tag is the analysis of the full stop, as every sentence end is represented as a full stop by the system, whether or not explicitly indicated in the sentence. The analysis of an unknown word is (ignoring the full stop info): \begin{small} \begin{alltt} ^genoma/*genoma\$ \end{alltt} \end{small}

\noindent and the analysis of an ambiguous word:

\begin{small} \begin{alltt} ^casa/casa<n><f><sg>/casar<vblex><pri><p3><sg>/casar<vblex><imp><p2><sg>\$\end{alltt} \end{small} Each lexical form (lemma plus morphological analysis) is presented as a possible analysis of the word \emph{casa}. \subsubsection{The tagger output} To know the output of the tagger for a source language text, type the following in the terminal (example for the Catalan-Spanish direction): \begin{small} \begin{alltt} echo "sabates" | apertium-destxt | lt-proc ca-es.automorf.bin \\|apertium-tagger -g ca-es.prob \end{alltt} \end{small} The output will be: \begin{small} \begin{alltt} ^sabata<n><f><pl>\$^./.<sent>\$[][] \end{alltt} \end{small} The output for an ambiguous word will be like the one above, since the tagger chooses one lexical form among all the possibilities. Therefore, the output for \emph{casa} in Catalan will be, for example (depending on the context): \begin{small} \begin{alltt} ^casa<n><f><sg>\$^.<sent>\$[][] \end{alltt} \end{small} \subsubsection{The \texttt{pretransfer} output} This module applies some changes to multiwords (move the lemma queue of a multiword with inner inflection just after the lemma head). To know its output, type: \begin{small} \begin{alltt} echo "sabates" | apertium-destxt | lt-proc ca-es.automorf.bin \\|apertium-tagger -g ca-es.prob | apertium-pretransfer \end{alltt} \end{small} Since \emph{sabates} is not a multiword, this module does not alter its input. \subsubsection{The structural and lexical transfer output} To know how a word, phrase or sentence is translated into the target language and processed by structural transfer rules, type the following in the terminal: \begin{small} \begin{alltt} echo "sabates" | apertium-destxt | lt-proc ca-es.automorf.bin \\|apertium-tagger -g ca-es.prob | apertium-pretransfer \\| ./ca-es.transfer ca-es.autobil.bin \end{alltt} \end{small} The output for this word will be: \begin{small} \begin{alltt} ^zapato<n><m><pl>\$^.<sent>\$[][] \end{alltt} \end{small} Analysing how a word or phrase is output by this module can help you detect errors in the bilingual dictionary or in the structural transfer rules. Typical bilingual dictionary errors are: two equivalents for the same source language lexical form, or wrong assignment of grammatical symbols. Errors due to structural transfer rules vary a lot depending on the actions performed by the rules. \subsubsection{The morphological generator output} To know how a word is generated by the system, type the following in the terminal: \begin{small} \begin{alltt} echo "sabates" | apertium-destxt | lt-proc ca-es.automorf.bin \\|apertium-tagger -g ca-es.prob | apertium-pretransfer \\| ./ca-es.transfer ca-es.autobil.bin | ltproc -g ca-es.autogen.bin \end{alltt} \end{small} With this command you can detect generation errors due to an incorrect entry in the target language monolingual dictionary or to a divergence between the output of the bilingual dictionary (the output of the previous module) and the entry in the monolingual dictionary. The correct output for the input \emph{sabates} would be: \begin{small} \begin{alltt} zapatos.[][] \end{alltt} \end{small} There are in this step no grammatical symbols, and the word appears inflected. \subsubsection{The post-generator output} It is not very usual to have errors due to the post-generator, because of its generally small size and the fact that it is seldom changed after adding usual combinations, but you can also test how a source language text comes out of this module, by typing: \begin{small} \begin{alltt} echo "sabates" | apertium-destxt | lt-proc ca-es.automorf.bin \\|apertium-tagger -g ca-es.prob | apertium-pretransfer \\| ./ca-es.transfer ca-es.autobil.bin | ltproc -g ca-es.autogen.bin \\| ltproc -p es-ca.autopgen.bin \end{alltt} \end{small} \subsubsection{The Apertium output} You can put all the modules of the system in the pipeline structure and see how a source language text goes through all the modules and gets translated into the target language. You just have to add the re-formatter to the previous command: \begin{small} \begin{alltt} echo "sabates" | apertium-destxt | lt-proc ca-es.automorf.bin \\|apertium-tagger -g ca-es.prob | apertium-pretransfer \\| ./ca-es.transfer ca-es.autobil.bin | ltproc -g ca-es.autogen.bin \\| ltproc -p es-ca.autopgen.bin | apertium-retxt \end{alltt} \end{small} This is the same as using the \texttt{apertium} shell script provided by the Apertium package: \begin{small} \begin{alltt} echo "sabates" | apertium . ca-es \end{alltt} \end{small} \noindent (The dot indicates the directory where the linguistic data are saved, in this case the current directory). Of course, instead of typing all the presented commands every time you need to test a translation, you can create shell scripts for every action and use them to test the output of each module. \subsection{Error examples} 1) We can get the following kind of output in a translation: \begin{small} \begin{alltt} \$ echo "nord" | apertium . ca-es \$#norte<n><m><sg> \end{alltt} \end{small} This means that the word was correctly translated by the bilingual dictionary but that the system does not find it in the Spanish morphological dictionary to generate it. The problem can be in the morphological dictionary but can also be caused by an incorrect bilingual entry, in which the grammatical symbols that the translated word is assigned do not correspond with the grammatical symbols that this word has in the morphological dictionary. 2) The following \texttt{es-ca} bilingual entry does not take into account the gender change between \emph{adhesiu} (masculine) and \emph{pegatina} (feminine), causing the translator to give an error: \begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>pegatina<\textbf{s} \textsl{n}="n"/></\textbf{l}> <\textbf{r}>adhesiu<\textbf{s} \textsl{n}="n"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} \begin{small} \begin{alltt} \$ echo "adhesiu" | apertium . ca-es \$#pegatina<n><m><sg> \end{alltt} \end{small} The correct entry should be: \begin{small} \begin{alltt} <\textbf{e}> <\textbf{p}> <\textbf{l}>pegatina<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="f"/></\textbf{l}> <\textbf{r}>adhesiu<\textbf{s} \textsl{n}="n"/><\textbf{s} \textsl{n}="m"/></\textbf{r}> </\textbf{p}> </\textbf{e}> \end{alltt} \end{small} 3) The following error is given when the source language lexical form can not be found in the bilingual dictionary, either because there is not an entry for this lemma or because the entry does not correspond with the grammatical symbols received from the analyser: \begin{small} \begin{alltt} \$ echo "illot" | apertium . ca-es \$@illot<n><m><sg> \end{alltt} \end{small} 4) When a source language lexical form has two correspondences in the bilingual dictionary, the translator output is like the following one: \begin{small} \begin{alltt} \$ echo "llavor" | apertium . ca-es \\$ #pepita<n>/semilla<n><m><sg> \end{alltt} \end{small}

The solution is to put a direction restriction in one of the bilingual entries.

Some errors can be due to structural transfer rules. The way to solve a problem whose origin we don’t know, is to test the output of the different modules to detect where the problem arises.

\subsection{Testing the integrity of the dictionaries}\label{integridad}

It is highly advisable to test the integrity of our dictionaries from time to time, especially if we changed them significantly –or if we changed the transfer rules, because some errors can be due to its application.

The test is carried out in one translation direction. For this reason, for a given language pair, you will have to perform two tests, one in each direction.

The steps you have to follow to perform the test are:

\begin{itemize}

\item expand the source language monolingual dictionary, using the \texttt{lt-expand} tool, to obtain all the lexical forms (which are the forms that appear on the right of the colon in the output file);

\item send these lexical forms (except those that are only generation forms, which \texttt{lt-expand} will have marked with the symbol ’\texttt{<}’ ) through all the system modules from pretransfer to the generator;

\item Search in the result, the lexical forms marked with the symbols ’\texttt{\#}’ , ’\texttt{@}’ or ’\texttt{/}’, which will be the error forms (see Section~\ref{subsec:marcaserror}).

\end{itemize}

\section{Generating a new Apertium system from modified data}

If you make changes to any of the linguistic data files of Apertium (dictionaries, transfer rules or tagger definition file), the changes will not be applied until you recompile the modules. To do this, type \texttt{make} in the directory where the linguistic data are saved so that the system generates the new binary files.

If changes were made to the tagger definition file or to the corpora used to train the tagger, you will need also to retrain the tagger: in the same linguistic data directory, you have to type (example for the Spanish tagger in the es-ca translator) \texttt{make -f es-ca-unsupervised.make} for unsupervised training or \texttt{make -f es-ca-supervised.make} for supervised training.

After compilation, \texttt{apertium} will already use the new data.