Annotating MWE s in the I rish UD Treebank (2020)

Abstract: This paper reports on the analysis and annotation of Multiword Expressions in the Irish Universal Dependency Treebank. We provide a linguistic discussion around decisions on how to appropri- ately label Irish MWEs using the compound, flat and fixed dependency relation labels within the framework of the Universal Dependencies annotation guidelines. We discuss some nuances of the Irish language that pose challenges for assigning these UD labels and provide this report in support of the Irish UD annotation guidelines. With this we hope to ensure consistency in annotation across the dataset and provide a basis for future MWE annotation for Irish.

## introduction the aim of the universal dependencies (ud) project (nivre et al., 2016) is to facilitate and improve cross-lingual learning and multilingual parsing through the creation of a harmonised set of annotation guidelines for treebanks across multiple languages. as the project and guidelines evolve, new language treebanks are being added at each 6 monthly release. the irish ud treebank (iudt) (lynn et al., 2017) has been part of the ud project since the v.1 release in 2015, following a conversion from the original irish dependency treebank (idt) @xcite (lynn, 2016). @xcite until recently, however, there was little opportunity to fully explore the treatment of multiword expressions (mwes) in either the original idt annotation scheme or the converted ud scheme. this is mainly due to two factors: (i) both treebanks were the product of a phd dissertation with limited scope, and (ii) prior research (both theoretical and applied) on mwes in irish was limited and generally insufficient in terms of natural language processing (nlp) relevance or application (see section 2). these factors are commonplace as challenges that face lowresource languages such as irish. mwes are reported to make up a large part of natural language, as much as forty percent of our mental lexicon (jackendoff, 1997; fellbaum, 1998). as constant et al. (2017) observe, providing an exact definition of mwes can be controversial and there are varying interpretations and analyses to be found. these variations may well be due to the differing motivations for the need for definition (theoretical, applied, etc.). our approach has been informed by the work of sag et al. (2002), baldwin and kim (2010) and ramisch (2015), whose works lie within the field of nlp. we define mwes to be a string of two or more tokens, which form a unit at a semantic, syntactic or lexical level. research into mwes, particularly with respect to nlp, has grown substantially since it has become increasingly apparent that they present a bottleneck for automatic processing of human language (sag et al., 2002). in fact, both the ict cost action parseme (ic1207) @xcite and the establishment of the mwe workshop series foot_2 were motivated by the need to establish how best to represent and encode mwes for the benefit of improving nlp across languages. syntactic parsing is one particular area that can see improvement in accuracy when additional information is known with respect to the use of multiword expressions in a language (nivre and nilsson, 2004; seretan, 2011; green et al., 2013; candito and constant, 2014; savary et al., 2015). essentially, a parsing system is expected to perform better when it is aware that a string of words should be treated as one syntactic unit instead of individual tokens (e.g. they tried to hold up a bank vs the container can hold up to 10 gallons). given the small size of the irish ud treebank, @xcite it makes sense that both improving the quality of the trees with additional information (such as accurately labelled mwes) and increasing the size of the dataset should be treated with similar levels of importance. this paper reports on the labelling of mwes in the v2.6 release of the irish ud treebank. a summary of our contribution is as follows: (1) we contribute to the quality of the v2.6 iudt release by fully reviewing and updating mwe annotation. (2) we propose an approach to analysing and labelling irish compounds, fixed expressions and flat proper noun strings within the ud framework. (3) in particular, we highlight the issues arising in differentiating between compound and nominal modifier (nmod) dependents while reporting on a small survey to address this, and hope that this may be helpful to other treebank developers who face similar challenges. while the ud guidelines aim to capture linguistic universals, there will always be language specific features to consider when meeting the trade-off between cross-lingual consistency and a sufficient representation of that language. given that this field of research is still relatively under-explored in irish, some annotation choices may still be controversial to those in the field of irish linguistics outside the ud framework, but nonetheless this starting point will be useful for any future studies in this area. ## related work as highlighted by losnegaard (2016) and parra escartín (2018), for a long time, most mwe research was traditionally based on major languages such as english. in terms of linguistic data annotation, analysis of mwes had only been carried out in a limited number of dependency treebanks (e.g. czech (bejček and straňák, 2010), hungarian (vincze et al., 2013) and turkish (eryiǧit et al., 2015)). in a step to address this, the ict cost action parseme 6 set out to draw up guidelines for classifying and categorising mwes across multiple languages. the main goal was to provide a framework for processing mwes in order to improve performance in the areas of machine translation and parsing (savary et al., 2015; savary et al., 2017; ramisch et al., 2018; losnegaard et al., 2016). @xcite complementary work (rosén et al., 2015; rosén et al., 2016) proposed general guidelines for annotating mwes in both constituency and dependency treebanks across 15 languages with the help of a focused survey. through this, it was observed that it should be possible to search for various types of mwes based on their characteristics (e.g. compositional vs non-compositional). in terms of syntactic parsing, candito and constant (2014) observe that while mwe information is intuitively supposed to help parsing, it is difficult to prove this in a realistic setting. difficulties arise when mwes are automatically identified -an approach which can result in error propagation (constant et al., 2012). it was also noted that the use of external lexicons did not seem to suffice in mwe processing, and that the use of data-driven external information would potentially help with this. this was supported by schneider (2014) and the various experiments carried out by constant et al. (2019) in assessing different approaches to mwe identification and parsing. constant and nivre (2016) developed a transition-based system which performed joint syntactic analysis and mwe identification, with promising results based on english and french. overall, the need for mwe-aware treebanks to appropriately train statistical parsers has therefore become more evident. with respect to ud treebank annotation, the language agnostic guidelines for mwes deal mainly with compounds, fixed expressions and strings such as proper nouns. the following labels are proposed in the ud v2 guidelines: foot_5 compound; fixed; and flat. however, as interpretation of mwes seems to vary across treebank development teams, a working group has been formed in the ud project to seek to harmonise the treatment of mwes across treebanks. @xcite some treebank groups have also reported on their specific approaches to dealing with mwes in their data (e.g. croatian (sojat and filko, 2016), english (schuster and manning, 2016), spanish (martínez alonso and zeman, 2016), turkish (sulubacak and eryiǧit, 2018) and farsi (qasemizadeh, 2014)). more recently, kahane (2018) provided an alternative proposal for treating idioms and fixed expressions through examining current french and english analyses in ud treebanks. the following research contributes to our understanding of mwes in irish. stenson (1981) discusses idiomatic constructions including verb-object constructions, verb-particle constructions and idiomatic constructions using the copula. in terms of theoretical studies, bloch-trojnar (2009) and bayda (2015) have both carried out some research on light verb constructions, ní loingsigh (2016) on idioms, and ó domhnalláin and ó baoill (1975) on verbal constructions with prepositions. the christian brothers (1999) offer a limited summary of compounds in irish, but without a specific discussion of mwes. uí dhonnchadha (2009) provides a summary on treatment of phrasal verbs in her work on part-of-speech tagging and a constraint grammar for irish. the output format of this pos-tagger resulted in some compounds, fixed expressions and proper noun strings being captured in earlier versions of the iudt (lynn and foster, 2016) -the annotation of which were reviewed in this study. in terms of categorisation, a definition of a taxonomy of mwes in irish is in its early stages of development (walsh et al., 2019). this work also discusses phrasal verbs in more detail through the lens of verb particle constructions and inherently adpositional verbs. our main sources of reference on irish grammar in our work are studies in irish syntax (stenson, 1981), the christian brothers' irish grammar (christian-brothers, 1999) and also online dictionaries such as an bunachar náisiúnta téarmaíochta don ghaeilge (the national terminology database for irish), @xcite the new english-irish dictionary, foot_8 and also the foclóir gaeilge-béarla and an foclóir beag which are available at the teanglann website. @xcite a valency dictionary for irish verbs has also been developed and made available on the pota focal website (foclóir briathra gaeilge), providing additional insight into the labelling of verb particles. @xcite ## categories of mwes in the iudt the irish universal dependency treebank (iudt) v2.5 (and earlier releases) contained many inconsistencies with respect to mwe annotations. this was mainly as a result of (i) automatic conversion from the idt annotation scheme (ii) changes in the ud guidelines and (iii) a lack of broad coverage research into irish mwes. in this section we highlight the various ud labels for mwe categories (compounds, fixed, and flat (nivre et al., 2020)) and how we have applied them to irish in the v2.6 release. these categories are generally tailored towards those mwe types that demonstrate syntactic idiosyncrasy, i.e. they do not conform to the normal syntactic behaviour of the language. as such, many categories dealt with elsewhere (e.g. the verbal mwes recognised in the parseme shared task) are not specifically addressed here. our discovery/labelling approach involved flagging potential mwes during treebank expansion annotation for the v2.6 release. based on patterns we observed, we then actively searched for candidates for review in the v2.5 version. compound labelling proved most challenging and therefore constitutes much of the discussion below. ## conclusion in this article we have reported on a review and update of mwe annotations in the irish ud treebank for the v2.6 ud release. we have provided our analysis and motivations for applying the compound, fixed and flat labels to irish mwes, and discussed the various challenges involved therein. in the v2.6 treebank of size 64,745 tokens, the compound label was applied 160 times (141 compound, 19 compound:prt), fixed was applied 950 times, and flat was applied 2252 times (1399 flat, 695 flat:name, 150 flat:foreign). while our approach is mostly in line with the ud annotation guidelines, we note that our use of the flat label is too broad as it also incorporates named entities (ne) in general. the opportunity for manual review of the treebank data allowed for previously unknown ne data to be captured easily. in the future, we want to remove the flat label in these cases and capture ne information in the misc column instead. finally, a note on inflected prepositions (see section 3.2). currently we do not split pronominal prepositions into adp + pron. if however, future versions of the treebank undergo changes with respect to splitting multi-token units (e.g. leis 'with it' → le + é), the uninflected preposition token could be considered part of a fixed expression (e.g. chomh maith le).