PARSEME Meets U niversal D ependencies: Getting on the Same Page in Representing Multiword Expressions (2023)

Abstract: Multiword expressions (MWEs) are challenging and pervasive phenomena whose idiosyncratic properties show notably at the levels of lexicon, morphology, and syntax. Thus, they should best be annotated jointly with morphosyntax. We discuss two multilingual initiatives, Universal Dependencies and PARSEME, addressing these annotation layers in cross-lingually unified ways. We compare the annotation principles of these initiatives with respect to MWEs, and we put forward a roadmap towards their gradual unification. The expected outcomes are more consistent treebanking and higher universality in modeling idiosyncrasy.

## introduction multiword expression (mwe) is an umbrella term spanning a range of linguistic phenomena whose common property is idiosyncrasy or, more specifically, idiomaticity, which may manifest in many different respects: lexical, morphological, syntactic, semantic, pragmatic, and statistical @xcite . mwes are challenging and pervasive. for instance, in an mwe-annotated corpus of french @xcite , over 11% of all tokens belong to mwes. moreover, mwes likely exist in any natural language. therefore, modeling idiosyncrasy in language resources and tools is a natural quest. this position paper addresses two language annotation frameworks, universal dependencies and parseme, from the point of view of mwes. universal dependencies foot_0 (ud; @xcite @xcite is a framework for consistent annotation of grammar (parts of speech, morphological features, and syntactic dependencies) across many languages. it is an open community effort with over 300 contributors producing nearly 200 treebanks in over 100 languages. parseme foot_1 @xcite is a scientific network which evolved from a homonymous cost action dedicated to parsing and mwes. one of its major outcomes is a multilingual corpus annotated for verbal mwes (vmwes) in 26 languages by over 160 native annotators. the common objective of ud and parseme is universality, i.e., the development of crosslinguistically consistent and applicable language descriptions. such consistency leads to valuable insights about linguistic phenomena (including idiosyncrasy), contributes to contrastive studies, and promotes progress in nlp across many languages. concretely, both ud and parseme (i) develop cross-lingually unified and continuously enhanced annotation guidelines, (ii) annotate, enhance, and release corpora on the basis of these guidelines, and (iii) use these corpora to develop nlp tools for syntactic parsing and mwe identification. despite their common goals, ud and parseme have operated relatively independently, ending up with partly divergent and competing terminologies and methods. some of the mwe types addressed by parseme, such as light-verb constructions, are annotated to some extent also within ud, but typically not consistently across languages, as we will discuss in section 3.6. we think it is desirable to keep morphosyntactic annotations separate from mwe-related annotations. 3 the desire for greater convergence between ud and parseme practices has steadily grown as the initiatives have matured. parseme has relied on the ud format (cf. sec. 3.2) and data in its latest corpus releases. in august 2021, a joint dagstuhl seminar on universals of linguistic idiosyncrasy in multilingual computational linguistics brought together the two initiatives @xcite . 4 finally, september 2022 saw the start of a new cost action entitled unidive (universality, diversity and idiosyncrasy in language technology), with ud/-parseme unification on the agenda. this paper aims at providing a roadmap towards this unification. we first survey the dimensions of mwe idiosyncrasy (sec. 2) and compare the two frameworks' annotation principles that bear on mwes (sec. 3). then, we offer short-, mid-and long-term proposals for adjusting the frameworks, paving the way towards eventually unifying them (sec. 4). sec. 5 concludes with future perspectives. 3 the current status led to problems for the vmwe identifiers evaluated in the parseme shared tasks @xcite , which were given ud morphosyntactic annotations as input, and were expected to predict vmwe annotations. since some mwe-related phenomena currently are annotated in the morphosyntactic layer, this type of evaluation is biased (since part of the information to be predicted is already given as input). given these examples, mwe idiosyncrasy can be considered along two orthogonal dimensions. occurrences vs. types some idiomatic properties of mwes display at the level of individual occurrences of mwes @xcite . conversely, others are visible at the level of types, that is, sets of surface realizations of the same mwe. for instance, the cranberry word (1), irregular agreement (2), and irregular syntax (3) can be observed in every single occurrence of these mwes. on the other hand, compulsory agreement (4) or restricted inflection (6) can only be attested while considering several possible surface realizations of the given mwe, so as to test whether different inflection, agreement or syntactic alternations do or do not preserve the idiomatic reading. @xcite propose a different but isomorphic terminology, contrasting restrictive vs. defective idiosyncrasy. a defective property excludes a literal interpretation of a given mwe. this is observable precisely at the level of individual mwe occurrences, as in ( 1 )-(3). a restrictive property reduces the number of possible surface realizations of a given mwe relative to the corresponding literal interpretation. this amounts to idiosyncrasy at the level of mwe types, as in ( 4 )-( 6 ). ## annotation principles this section compares the annotation principles of ud and parseme, focusing on mwes. ## towards ud-parseme unification the discrepancies discussed above harm universality, therefore we are taking steps towards unifying ud and parseme. the expected advantage lies in a better parallelism in annotating syntactic vs. semantic and regular vs. idiosyncratic properties. our intuition is that semantic non-compositionality is an intriguing phenomenon and annotators wish to signal it even when annotating morphosyntax. if an mwe has (partly) regular syntax but idiomatic semantics, and if only morphosyntactic labels are available, annotators might prefer to signal idiosyncrasy rather than regularity. 12 another temptation is to introduce new subtypes such as obj:lvc, which could block other useful syntactic distinctions that could be encoded with subtypes (since recursive subtypes are not allowed). adding the mwe layer to the annotation schema solves these problems. another motivation is that both automatic processing of mwes and parsing benefit from solving the two tasks jointly @xcite , therefore aligning morphosyntactic and mwe annotations serves nlp. here, we lay out a multistage unification roadmap for major issues, summarized in appendix b. note that no re-annotation effort is required on the ud side in the first two stages. this is important for at least three reasons. firstly, while for parseme idiosyncrasy is central, for ud it is only one of the many phenomena to be modeled. it is therefore natural for the parseme community to be the main responsible party for changes related to idiosyncrasy. secondly, the ud community of treebank creators and users is very large. any change in annotation principles, in order to be widely adopted, should minimize manual reannotation and should be divided into small, easily achievable steps. thirdly, as mentioned in section 3.1, while parseme annotation is sporadic, ud trees fully cover the annotated text, which implies a heavier (re-)annotation workload. ## conclusions and future work we have compared how ud and parseme capture linguistic idiosyncrasy. since parseme largely agrees with ud's objectives, it increasingly follows ud on data formats, morphology, (regular) syntax and tokenisation. we are optimistic about ud and parseme joining forces for compatible encoding of regular and idiosyncratic phenomena, as detailed in our roadmap proposal. in the long run, these efforts might benefit from more typological insights. also, extending the annotation schema to large classes of constructions would enable an even more comprehensive account of idiosyncrasy. the implementation of these suggestions will depend, however, on a delicate balance between existing and upcoming data, automation tools, and-above all-on availability and willingness of contributors.