Agata Savary
email: agata.savary@universite-paris-saclay.fr

Verginica Barbu Mititelu

Nathan Schneider
email: nathan.schneider@georgetown.edu

Carlos Ramisch
email: carlos.ramisch@lis-lab.fr

PARSEME Meets Universal Dependencies: Getting on the Same Page in Representing Multiword Expressions

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. In this position paper 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 [START_REF] Baldwin | Multiword Expressions[END_REF].

MWEs are challenging and pervasive. For instance, in an MWE-annotated corpus of French [START_REF] Candito | A French corpus annotated for multiword expressions and named entities[END_REF], 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 1 (UD; [START_REF] Nivre | Universal Dependencies v1: a multilingual treebank collection[END_REF][START_REF] Nivre | Universal Dependencies v2: An evergrowing multilingual treebank collection[END_REF][START_REF] De Marneffe | Universal Dependencies[END_REF] 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 2 [START_REF] Savary | PARSEME multilingual corpus of verbal multiword expressions[END_REF][START_REF] Ramisch | Edition 1.2 of the PARSEME Shared Task on Semi-supervised Identification of Verbal Multiword Expressions[END_REF] 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 [START_REF] Baldwin | Universals of Linguistic Idiosyncrasy in Multilingual Computational Linguistics[END_REF]. 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 [START_REF] Ramisch | Edition 1.2 of the PARSEME Shared Task on Semi-supervised Identification of Verbal Multiword Expressions[END_REF], 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). In (4), the possessive her must agree with the subject, otherwise the MWE is understood literally, as in (5).

(4) She knows her stuff.

(en)

'She is skilled. '

(5) #She knows my stuff (en)

Concrete nouns in verb-object constructions can inflect for number, but pluralizing the noun in (6) implies losing the idiomatic reading, as shown in ( 7). 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 [START_REF] Savary | Without lexicons, multiword expression identification will never fly: A position statement[END_REF]. 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. [START_REF] Timm Lichte | Lexical encoding formats for multi-word expressions: The challenge of "irregular" regularities[END_REF] 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).

Morphosyntactic vs. Semantic Idiosyncrasy

The idiosyncratic properties discussed above occur at the morphosyntactic level. However, the most salient property of MWEs is semantic noncompositionality: their meaning cannot be deduced from the meanings of their components and from their syntactic structure in a way deemed regular [START_REF] Sag | Multiword expressions: A pain in the neck for NLP[END_REF]. Examples (1)-( 6) can safely be considered as semantically non-compositional.

Distinguishing morphosyntactic from semantic idiosyncrasy is a hard nut to crack. First, the borders between morphology, syntax and semantics are fuzzy. For instance, the notions of syntactic and semantic arguments are closely related in the linguistic debate about arguments vs. adjuncts [START_REF] Przepiórkowski | Arguments and adjuncts in Universal Dependencies[END_REF]. Second, idiosyncratic properties in MWEs usually cross multiple layers of linguistic description. For instance, the MWE in (3) exhibits not only unusual syntax but also restricted inflection, as in (6). Third, semantic non-compositionality is hard to test directly and reliably at the level of occurrences. Nonetheless, it can be more accurately approximated by lexical and morphosyntactic inflexibility, by testing it at the level of types [START_REF] Gross | Degré de figement des noms composés[END_REF][START_REF] Gibbs | Psycholinguistic studies on the syntactic behavior of idioms[END_REF]. This again suggests that morphosyntactic and semantic idiosyncrasies are entangled. [START_REF] Sylvain Kahane | Multi-word annotation in syntactic treebanks -Propositions for Universal Dependencies[END_REF] propose considering syntactic and semantic idiosyncrasy as separate dimensions. They consider: (i) regular constructions, subsystems and irregular constructions, (ii) compositional, semi-compositional and non-compositional expressions, along the syntactic and semantic axes. Various expressions are then placed in this twodimensional space. For instance, syntactically irregular constructions can be semantically compositional, e.g. (fr) peser lourd (lit. 'to weigh heavy') 'to be very heavy'. While this classification is promising, it fails to provide an operational definition of semantic non-compositionality. In particular, assuming that formal semantics accurately approximates semantic compositionality, there can be no constructions with irregular syntax but compositional semantics. 6 Still, what we retain from [START_REF] Sylvain Kahane | Multi-word annotation in syntactic treebanks -Propositions for Universal Dependencies[END_REF] is the premise that syntactic and semantic properties of MWEs should be annotated at different layers as much as possible. In particular, it is useful to display regular syntax in MWEs despite their semantic idiosyncrasy.

YES NO

Given In short, we distinguish occurrence vs. type and lexical/morphosyntactic vs. semantic idiosyncrasies in MWEs, but we note that these dimensions are closely linked, as shown in Fig. 1. First, if at least one MWE occurrence is idiosyncratic, then the whole type is irregular. Second, lexical and/or morphosyntactic idiosyncrasy of MWE occurrences and/or types approximates their semantic non-compositionality. Note that the choice of testing defectiveness (of an occurrence) before restrictiveness (of the whole type) is not arbitrary. First, basic observable units in an annotated corpus are occurrences (by contrast, lexicons primarily focus on types). Second, testing irregularity for an occurrence is cognitively easier than regarding the whole type. Third, the definition of a restrictive property is based on the understanding of the literal interpretation of a potential MWE. However, if a token is defective, its literal interpretation is excluded. Finally, the border between defective and restrictive properties is precisely where we would like to ultimately draw the line between UD and PARSEME annotations, i.e. only defective properties would be rendered in the UD layers.

Annotation Principles

This section compares the annotation principles of UD and PARSEME, focusing on MWEs.

Objectives and Principles

The common objective of UD and PARSEME is universality, defined as development of crosslinguistically consistent and applicable language descriptions. 7 Both initiatives aim at representing in a unified way those phenomena which are truly similar, while leaving room for language-specific categories, relations and guidelines. The utility of these descriptions is twofold -meaningful linguistic analysis and useful language processing -in both monolingual and cross-lingual settings.

UD descriptions concern several aspects of language: segmentation, lemmas, morphology and syntax. According to the annotation properties defined by [START_REF] Mathet | The unified and holistic method gamma (𝛾) for inter-annotator agreement measure and alignment[END_REF], these descriptions include unitizing (identify sentence and word boundaries) and have a full covering (concern all words in a corpus). PARSEME descriptions are mostly semantic (even if largely approximated by morphosyntax, see below). They also require unitizing, but are sporadic (only focus on components of MWEs), can be 7 This is in contrast with the quest for absolute language universals (Greenberg, 1966;[START_REF] Chomsky | Reflections on Language[END_REF][START_REF] Tallerman | If language is a jungle, why are we all cultivating the same plot?[END_REF]. nested ([[let] 2 the cat [out] 2 of the bag] 1 'reveal a secret') and exhibit free overlap (take 1,2 a walk 1 and a shower 2 ).

Notations and Formats

With respect to data formats, UD and PARSEME are largely compatible. Consider the example in sentence (8). Its main UD and PARSEME annotations are visualized in Fig. 2: parts of speech and dependencies are the UD-specific data, while MWEs (highlighted in boldface and subscripts) are tagged by PARSEME. The same example, in more detail, is presented in Fig. 3

Words and Tokens

Word is a fundamental notion both for UD, since its basic annotation unit is a word, and for PARSEME, since MWEs must contain at least two words. However, defining a word is one of the hardest challenges in UD, due to its fuzzy borders with morphemes on the one hand and with MWEs on the other. In UD, words are defined in morphosyntactic terms as units bearing morphological properties (e.g. a single POS) and entering into syntactic relations. Words do not always coincide with orthographic units called tokens. 9 Therefore, UD defines a 3-fold relationship between words and tokens:

• A token coincides with a word.

• Several tokens build up one multitoken word (MTW), as in 20 000. • One multiword token (MWT) contains several words, as in (fr) aux (à+les) 'in.the'.

The words (not orthographic tokens) form the basic units of analysis and receive integer indices. MWTs are represented as spans over multiple words (e.g. 4-5 in Fig. 3), including cases where words (à and le) are not retrievable from tokens (au). PARSEME conforms to the same definitions of words, MWTs, and MTWs, with implications for MWEs like in Fig. 3. Only the adposition à 'to' belongs to the MWE; 10 the determiner le 'the' is excluded. This is possible in PARSEME due to splitting MWTs into words by UD. Still, PARSEME covers a considerably higher number of MWTs than UD, especially verb-particle constructions written sometimes as 1 and sometimes as 2 tokens as in (9), and orthographically unitary (closed or synthetic) compounds as in (10).

(9) auf-passen, on-fit.inf,

pass fit.imp auf! on! (de)
lit. 'to fit on, fit on!' 'to be careful, be careful!' 9 Neither UD nor PARSEME define tokens. We see them as units stemming from segmenting raw text for annotation. 10 As evidenced by variants like (fr) voler à son secours (lit. 'to.fly to his/her rescue') 'to hurry up to help him/her' This discrepancy leads to two issues, illustrated in Fig. 4. First, the definition of a word is inconsistent: item 3 is one word for UD but two words for PARSEME. Second, in item 11 only rolle 'role' belongs to an MWE, since Haupt 'head' can be freely replaced (Nebenrolle spielen 'play the secondary role'). This cannot be rendered if UD keeps compounds unsplit.

Morphology and Syntax

In UD, the morphological description of a word employs 17 universal POS tags and over 200 values for morphological features (columns 4 and 6 in Fig. 3), though explicitly admitting that some of them may not be necessary in some languages. Syntactic annotation in UD follows the dependency approach and adopts the lexicalist principle. Namely, words are divided into content words -typically verbs, nouns, adjectives or adverbs, with referential meaningand function words -determiners, adpositions, auxiliaries, etc. Content words are linked by syntactic relations, while function words attach to the content words they modify. For instance, in Fig. 2, the verb is the head of the auxiliary (items 2-3) and the nouns are the heads of the prepositions (items 4-6 and 7-8) rather than vice versa. A set of 37 syntactic relations considered universal (column 8 in Fig. 3) is defined. More specific relations in a language are accepted as subtypes of the universal ones (e.g. obl:arg in line 6 in Fig. 3) and 26 such subtypes are currently found in the UD treebanks. Treebanks are not required to use language-specific extensions, even if they cover phenomena for which such extensions are defined. This leads to significant inconsistencies in the use of subrelations, even among treebanks of the same language.

PARSEME, while modeling idiosyncrasy, tries to remain as independent of a particular linguistic framework as possible. It considers, for instance, that in a prepositional phrase a preposition directly governs a noun, or the opposite, depending on a particular linguistic theory. However, PARSEME approximates semantic compositionality by lexical and morphosyntactic flexibility tests that are driven by syntactic structure. Thus, the main PARSEME decision diagram asks questions about the syntactic head of the candidate expression, its dependents, its morphosyntactic category, etc. This implies a strong dependence on the underlying syntactic framework, and UD provides such framework, validated across many languages.

Another advantage for PARSEME is that the lexicalism in UD helps keep the MWE definition relatively simple. Namely, MWE components more easily form a weakly connected dependency graph (Sec. 3.5) if content words head function words than vice versa [START_REF] Savary | Polish corpus of verbal multiword expressions[END_REF]. One minor disadvantage from lexicalism concerns MWEs with copulas. For instance in (en) to be somebody 'to be important' the pronoun heads the copula be, which prevents PARSEME from saying that a verbal MWE is always headed by a verb.

The universality of UD thus enables universality for PARSEME, which has been increasingly relying on UD. For all 14 languages in version 1.2 of PARSEME, MWE annotations build upon UDcompatible corpora (manually annotated or automatically predicted); and among all 26 PARSEME corpora, 20 are UD-compatible.

The Notion of MWE

The way UD and PARSEME understand the notion of an MWE is the major source of apparent discrepancies between the two frameworks. UD did not attempt to formally define MWEs, using it as an umbrella term for expressions for which other syntactic relations seem useless or inconvenient. UD defines 3 dependency labels in such cases.

[fixed] is used for highly grammaticalised expressions, as in Fig. 5a, that typically behave as function words or short adverbials, i.e. belong to closed grammatical categories. The name of the label inspired by [START_REF] Sag | Multiword expressions: A pain in the neck for NLP[END_REF] signals morphosyntactic fixedness. By convention, all parts of such an expression are attached to the leftmost component, that is, the whole is considered headless (even if a head might be identifiable).

[flat] is meant for headless semi-fixed expressions, like names or complex numerals, as in Fig. 5b.

These belong to open categories and are subject to high productivity.

[compound] marks any word-level compounding, including nouns, adjectives, and verbs. Compounds are seen as headed expressions, i.e. modification relations are rendered, as in Fig. 5c. A compound may or may not be semantically compositional, as in Fig. 5c and5d, respectively.

This typology concerns dependency relations, not expressions. In particular, various labels can be mixed within one expression, as shown in Fig. 6. Some UD subtypes (e.g. compound:lvc, expl:pv) are related to MWEs in PARSEME (Sec. 3.6).

For PARSEME, an MWE is a combination of words with at least two lexicalized components (always realized by the same lexemes) displaying lex- For instance, the candidate on the left of Fig. 7 does not fulfill the conditions, but transforming it into the canonical form on the right restores graph connectivity and verb-headedness.

MWE Categories

PARSEME defines 3 quasi-universal categories (the first 3 below, present in many languages but not all), and 2 universal ones (the last 2 below, present in all languages under study). 11 Statistics about these annotations in the data are given in Appendix A.

Inherently Reflexive Verbs (IRV) combine a verb 𝑉 and a reflexive clitic 𝑅 such that (i) 𝑉 never occurs without 𝑅, as in (sv) gifta sig (lit. 'getmarried oneself') 'get married', or (ii) 𝑅 distinctly changes the meaning or valency of 𝑉 , as in (es) recogerse 'go home'/recoger 'gather'. They are contrasted with regular reflexives: true reflexive, reciprocal, middle passive and impersonal, e.g. (ro) casele se vând bine (lit. 'houses sell themselves well') 'houses sell well'. In UD, the above uses are divided into two classes, depending on if the reflexive clitic can or cannot be mapped on a semantic argument of the verb. In the former case, the "regular" dependency label corresponding to the role of the clitic is used, e.g. obj in (pl) myć się 'wash oneself'. The latter is covered by the expletive label. Subrelations can further distinguish these uses, in 11 https://parsemefr.lis-lab.fr/ parseme-st-guidelines/1.3/?page=030 _ Categories _ of _ VMWEs particular, expl:pv covers case (i) above, signaling idiosyncrasy.

Verb-Particle Constructions (VPCs) have two subclasses in PARSEME. In fully noncompositional VPCs (VPC.full), adding the particle considerably changes the meaning of the verb, as in (sv) Det gick upp för mig (lit. 'It went up to me') 'It occurred to me'. In semi-compositional VPCs (VPC.semi), the particle adds a partly predictable but non-spatial meaning to the verb, as in (sv) äta upp (lit. 'eat up') 'finish eating'. Verb-particle combinations where the particle only adds spatial meaning are not annotated, as in (sv) gick upp på vinden 'went up to the attic'. In UD the subrelation compound:prt can be used to connect a particle to its head verb, regardless of idiomaticity, i.e. all 3 examples above fall into this category.

Multi-Verb Constructions (MVCs) are idiomatic combinations of two verbs, e.g. (fr) laisser tomber (lit. 'to let fall') 'to abandon', in particular serial verbs in Asian languages, e.g. (hi) kar le (lit. 'do take') 'do for one's own benefit'. This relates to the UD compound:svc subrelation, which however covers serial verbs both in idiomatic and compositional uses, e.g. (ja) naguri korosi (lit. 'punch kill') 'kill by punching'.

Light-Verb Constructions (LVCs) are combinations of semantically light verbs and predicative nouns expressing the semantics of the action or state. Two subcategories are defined. In LVC.full the verb's subject is the noun's semantic argument as in (sl) imeti predavanje 'give a lecture'. In LVC.cause the verb's subject is the cause or source of the noun, as in (en) grant right. In UD, the same expressions are most often annotated with the "regular" obj dependency, even if the scope of the compound:lvc subrelation is similar to LVC.full.

Verbal Idioms (VID) is the most diverse category in PARSEME, gathering cases not covered by other categories. The verb's dependents are unrestricted, including subjects, as in (en) a little bird told me, direct objects, as in (6), etc. The verb can have several dependents, as in (en) cut a long story short, or combine features from other VMWE categories, as in (sv) sätta sig upp mot någon (lit. 'sit oneself up against someone') 'defy someone'. A VID candidate must display lexical or morphosyntactic idiosyncrasy, as in ( 1)-( 6). As VIDs are so diverse, there is no direct correspondence in UD. They are typically annotated as syntactically regular, possibly with subrelations for particles and reflexives when those are parts of the VID. The UD fixed relation cannot be used to signal inflexibility in VIDs since it is limited to functional 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 [START_REF] Constant | Multiword expression processing: A survey[END_REF][START_REF] Taslimipoor | MTLB-STRUCT @PARSEME 2020: Capturing unseen multiword expressions using multi-task learning and pre-trained masked language models[END_REF], 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 Sec-12 E.g. in the Romanian Reference Treebank, VIDs with regular syntax like avea loc 'take place' are marked as fixed.

# global.columns = ID FORM LEMMA . . . PARSEME:MWE 1 die der . . . * 2 Hauptrolle Hauptrolle . . . 1@6-10:LVC.full 3 spielen spielen . . . 1

Figure 8: PARSEME tag with a sub-token span tion 3.1, while PARSEME annotation is sporadic, UD trees fully cover the annotated text, which implies a heavier (re-)annotation workload.

Words and Tokens

PARSEME's notion of word is sometimes more granular than UD's (Sec. 3.3), and the segmentation of tokens into words would need to be reconciled. This is crucial for cases where MWEs cover parts of tokens, as in ( 10). Another such case occurs in Korean UD treebanks [START_REF] Chun | Building Universal Dependency treebanks in Korean[END_REF][START_REF] Tae Hwan Oh | Analysis of the Penn Korean Universal Dependency Treebank (PKT-UD): Manual revision to build robust parsing model in Korean[END_REF], where agglutinative postpositions are considered to form a syntactic word with their stem (segmented only in the lemma), as in ( 11). The postposition 에 (-ey) and following word 대해 (tayhay) together mean 'about', but because the postposition is not split, we would need to refer to a subword unit.

(11) 언어에 language:postp

대해 about 읽다 read (ko)
'read about languages'

Short-term Proposal

We propose to supplement existing UD parses with MWE annotations, without altering tokenization. MWEs that encompass entire MWTs, as in (9), are already covered by PARSEME. For cases like (10) and ( 11), the MWE column could specify sub-token spans, as in Fig. 8.

Long-term Proposal Parts of unsplit tokens participating separately in MWEs suggest a deficiency in UD's implementation of MWTs. We propose that, ultimately, UD syntactically recognize synthetic compounds as productive, regardless of the MWE status. This would require UD treebanks in some languages to systematically split current compound words into MWTs, ensuring each component word has an appropriate lemma and morphological features, and adding a dependency relation (such as compound or compound:prt) between them. This could also help disambiguate the interpretation of some compounds, as in (sv) bildrulle: bil+drulle, bild+rulle 'car maniac (bad driver), picture roll (roll Northern European Journal of Language Technology of film)'.13

Terminology and Guidelines

A common understanding of MWE-related terminology is a basic requirement for UD/PARSEME convergence. This could be achieved progressively.

Short-term proposals Different interpretations of the term "multiword expression" are understandable (Sec. 3.5), since it literally means an expression containing two or more words, with no further restriction. However, the term, as understood by the MWE community [START_REF] Baldwin | Multiword Expressions[END_REF], has an extra meaning component of idiosyncrasy (it is itself an MWE!), and we propose to adhere to this definition.14 This would mean, for UD, not to use the term MWE for phenomena considered regular, replacing the MWE heading (currently describing compound, fixed and flat) with a more neutral description like "other complex constructions". This should be easy to achieve, as MWEs do not have a technical definition in UD: the term is used casually in the guidelines, but is not part of the morphological or syntactic labels or their criteria. This proposal is conservative in the sense that it does not, in principle, require modifications of the annotations. 15 On the PARSEME side, the VPC label might be renamed to IVPC (for idiomatic VPC), so as to signal that verb-particle combinations can be both regular and idiomatic, and only the latter are MWEs (Sec. 3.6). Criticism of the current VMWE guidelines [START_REF] Savary | Polish corpus of verbal multiword expressions[END_REF][START_REF] Fotopoulou | Where Do Aspectual Variants of Light Verb Constructions Belong[END_REF] should also be addressed.

Mid-term Proposals A major mid-term requirement for PARSEME would be to extend its terminology and guidelines to all syntactic types of MWEs, rather than VMWEs only, e.g. based on the foundational work by [START_REF] Schneider | Comprehensive annotation of multiword expressions in a social web corpus[END_REF] and [START_REF] Candito | A French corpus annotated for multiword expressions and named entities[END_REF]. Challenges include defining the borders between named entities and MWEs.

Long-term Proposals Most languages contain productive grammatical subsystems which yield expressions with particular syntax and semantics, such as names, numbers, measurements, and dates [START_REF] Sylvain Kahane | Multi-word annotation in syntactic treebanks -Propositions for Universal Dependencies[END_REF][START_REF] Schneider | Mischievous nominal constructions in Universal Dependencies[END_REF]. Their heavy semantic load makes them central units of interest for NLP. They partially overlap with regular syntax and MWEs, e.g. (pl) Małgorzata Kowalska is a name with a regular noun-adjective structure, and (en) "Always Look On The Bright Side Of Life" is a title containing a VID. However, they also follow specific patterns, such as defective number agreement in (en) two million, and nesting (Fig. 6). They call for normalisation standards like TimeML and AMR [START_REF] Pustejovsky | TimeML: Robust specification of event and temporal expressions in text[END_REF][START_REF] Banarescu | Abstract Meaning Representation for sembanking[END_REF]. Annotating subsystems jointly with UD and PARSEME would require new instantiations of CONLL-U Plus, with extra columns, such as 'NE' in Fig. 9. Other initiatives are making progress towards adding entity and coreference layers to UD [START_REF] Nedoluzhko | CorefUD 1.0: Coreference meets Universal Dependencies[END_REF].

Occurrence vs. Type Encoding

We suggest unification steps towards a better account of the type/occurrence nature of idiosyncrasies.

Mid-term Proposals As soon as PARSEME extends its guidelines to all syntactic MWE types, they should be applied to all PARSEME corpora. The general principle would be: The PARSEME layer might deal with signaling total (rather than partial) fixedness if needed. The example in Fig. 5a would be annotated as in Fig. 9, depending if it is seen as analysable (lines 1-4) or headless (lines 11-14). The example in Fig. 5b would also be headless (lines 31-33), with a possible named entity type (column 12), if a subsystem layer is added to the schema (Sec. 4.2). These would be major changes, and authors of some treebanks might not be sufficiently interested in idiomaticity to accept the addition of a column. In this case, the previous distinction between fixed and flat should be kept to distinguish grammaticalized and productive headlessness. Subrelations such as compound:prt and compound:svc should probably be kept but used more consistently, since they are orthogonal to idiosyncrasy. Subrelations :lvc and :pv are superfluous: we propose to abandon them and use the 11th column instead.

Long-term Proposals Most optimally, the occurrence-type dichotomy of idiosyncrasy could be modeled in a framework in which corpus and lexicon are interlinked. A corpus would document occurrences, i.e. MWE occurrences would only be annotated for individual properties (including occurrence-wise idiosyncrasy such as irregular syntax). The lexicon would describe types, i.e. all occurrences of the same MWE would be linked to a lexicon entry representing its type and storing its type-wise properties such as categories (LVC, VID, etc.) and a meaning. A similar schema was implemented by [START_REF] Bejcek | Annotation of multiword expressions in the Prague depen-16[END_REF]. An MWE lexicon entry could also contain other type-specific properties such as canonical forms (lemmas), flexibility and agreement constraints, as in (4-6), and links to ontologies [START_REF] Hajnicz | Connections between the semantic layer of Walenty valency dictionary and PlWordNet[END_REF].

Finally, an MWE lexicon could be more compliant with a typological perspective. PARSEME's current MWE typology is driven by annotation needs, i.e. new categories are introduced if specific tests are needed to identify some MWE in texts. An orthogonal, more typologically-driven categorization could use cross-linguistic constructions and language-specific structural types [START_REF] Koptjevskaja-Tamm | Adnominal possession in the European languages: form and function[END_REF].

Data Quality

Both UD and PARSEME provide contributors with automatic data quality checkers. These should be unified and extended. PARSEME might enhance its validator to check compliance with guidelines (e.g. a verb in an LVC must have a single lexicalised dependent), and should integrate it with the UD validator, which runs automatically when a new version of a treebank is pushed to the GitHub repository. UD might develop tools inspired by PARSEME's consistency checks, in which a "vertical" view of the corpus groups annotations of the same MWE. This might help overcome inconsistencies within a treebank or within treebanks for the same language, e.g. due to the optionality of subrelations.

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.

Northern European Journal of Language Technology

A Statistics of the Use of MWErelated Labels in the UD and PARSEME Corpora

Table 1 shows the statistics and comments about the use of MWE-related labels in the UD treebanks in version 2.9 (with 131 treebanks in total). Table 2 documents the number of PARSEME languages in which the MWE labels are used. Table 3: Roadmap for the UD-PARSEME unification. Actions with white background require no manual (re-)annotation. Actions highlighted in blue will require major annotation effort: those in dark blue apply to all languages, whereas those in light blue (concerning subword-level annotations) apply to a subset of languages.

B Roadmap for UD-PARSEME Unification

Figure 2 :

 2 Figure 2: Sentence (8) with main annotations from UD (tree, POS tags) and PARSEME (bolding, subscripts).

  in the tabular .cupt format. 8 Each word is described in a separate line, with 11 tab-separated fields, whose headings are listed in the first line of each file. The first 10 columns are those of the .conllu format used by UD. The 11th column (PARSEME:MWE) is used by PARSEME. Components of MWEs annotated in column 11 are shown in bold.

Figure 4 :

 4 Figure 4: PARSEME annotation of unsplit MWTs.

Figure 6 :

 6 Figure 5: A complex preposition, a proper name (in Portuguese) and a nominal compound.synana

  # global.columns = ID FORM LEMMA UPOS XPOS FEATS HEAD DEPREL DEPS MISC PARSEME:MWE Annotation of sentence (8) as the first sentence in a corpus, in the .cupt format.

	1 Elle	il	PRON _ Gender=Fem|Number=Sing|Person=3	3 nsubj _ _ *
	2 a	avoir AUX	_ Mood=Ind|Number=Sing|Person=3|. . .	3 aux	_ _ *
	3 volé	voler VERB _ Gender=Masc|Number=Sing|Tense=Past|. . . 0 root	_ _ 1:VID
	4-5 au	_	_	_ _	_ _	_ _ *
	4 à	à	ADP	_ _	6 case	_ _ 1
	5 le	le	DET	_ Definite=Def|Gender=Masc|Number=Sing|. . . 6 det	_ _ *
	6 secours secours NOUN _ Gender=Masc|Number=Sing	3 obl:arg _ _ 1
	7 de	de	ADP	_ _	8 case	_ _ *
	8 Max	Max	PROPN _ _	6 nmod _ _ *
	Figure 3:

  Northern European Journal of Language Technology visits which I wanted to pay ⇒

	acl:relcl	xcomp	obj
			I wanted to pay visits
	Figure 7: A VMWE candidate and its canonical form.
	ical, morphological, syntactic or semantic idiosyn-
	crasies (Sec. 2). Even if PARSEME's ambition is to
	model MWEs in general, its major efforts were put
	into verbal MWEs (VMWEs). A VMWE is defined as
	an MWE whose canonical form (least syntactically
	marked form keeping the idiomatic reading) is such
	that its syntactic head is a verb, its other lexical-
	ized components form phrases directly dependent
	on this verb (the whole forms a weakly connected
	graph), and it passes the idiosyncrasy tests defined
	in the PARSEME guidelines. MWE candidate se-
	quences must be transformed into canonical forms.

  This would require a systematic use of the .cupt format to jointly represent all dimensions of idiosyncrasy. This would also question the utility of UD's fixed label, since fixedness is a property of types rather than occurrences. Maybe this label could be merged with flat and both renamed to headless to avoid confusion with previous interpretations.

	# global.columns = ID FORM . . . HEAD DEPREL . . . MWE NE
	1 Leave . . . 0 root	. . . *	*
	2 in	. . . 3 case	. . . 1:AdvMWE.fixed *
	3 case . . . 1 obl	. . . 1	*
	4 of	. . . 5 case	. . . *	*
	. . .
	11 Leave . . . 0 root	. . . *	*
	12 in	. . . 15 case	. . . 1:AdvMWE.fixed *
	13 case . . . 12 headless . . . 1	*
	14 of	. . . 12 headless . . . *	*
	31 a	. . . 32 det	. . . *	*
	32 TV	. . . 34 nsubj	. . . *	1:ORG
	33 Globo . . . 32 headless . . . *	1
	Figure 9: Two possible annotations for a multiword
	preposition; and a headless organization name.
				• UD layers only account for lexical/mor-
				phosyntactic idiosyncrasy of MWE occur-
				rences, such as irregular syntax in (3). Gram-
				matically regular MWE occurrences would
				receive "ordinary" annotation, regardless of
				semantics.
				• The PARSEME layer would signal any kind of
				semantic idiosyncrasy, i.e. it would flag each
				expression which is lexically/morphosyntac-
				tically irregular, whether at the level occur-
				rences or of types, e.g. for all examples in
				Sec. 2.

prt

  Table3summarises the proposals from Section 4. is used when the particle is not spacial, and advmod otherwise. The same distinction is suggested in the universal guidelines. Inconsistently used in Persian-Seraji vs. Persian-PerDT Inconsistently used in Turkish-BOUN and Turkish-IMST vs. all other Turkish treebanks

	Label	Treebanks Comments
	fixed	109	Limited to functional MWEs
	flat	119	Productive headless constructions
	compound	99	Productive headed compounds. 10 additional treebanks have the com-
			pound relation, but always with a subtype.
	expl:pv	20	Inconsistent use in Spanish-AnCora vs. Spanish-GSD, French-GSD vs.
			other French treebanks
	compound:prt In English compound:compound:svc 32 8
	compound:lvc	11	Most often commuted for obj.
			Northern European Journal of Language Technology

Table 1 :

 1 Use of MWE-related labels in the UD treebanks in version 2.9 (with 131 treebanks in total)

	Label	Corpora Comments
	IRV	8
	VPC.full	6	Greek and Hebrew use only VPC.full
	VPC.semi	5	Chinese uses only VPC.semi
	MVC	7
	LVC.full	14	Hindi allows adjectives in place of nouns
	LVC.cause	13	Not in Turkish
	VID	14

Table 2 :

 2 Use of MWE-related labels in the PARSEME corpora in version 1.2 (with 14 languages in total)

		Short-term	Mid-term	Long-term
		Assume idiosyncrasy of MWEs
	UD	Don't use MWE as umbrella term for

fixed, compound and flat

  Use the .cupt format Merge

fixed with flat, maybe rename to headless Abandon compound:lvc and expl:pv

				Annotate subwords whenever
				appropriate (e.g. Haupt-rolle)
				Extend the annotation schema to
			In new annotations, only flag token	subsystems
			idiosyncrasy
		Tag spans for subtokens (Hauptrolle)	Guidelines for all syntactic types	Link corpora with MWE lexicons,
	PARSEME	Rename VPCs to IVPCs	of MWEs, with subtypes for totally fixed MWEs Define the border between	encode MWE type properties in the lexicons Use orthogonal typology-inspired
			named entities and MWEs	categories
			Annotate MWEs of all syntactic types	Extend the annotation schema
			Flag both token and type idiosyncrasy	to constructions

Examples follow the PMWE conventions[START_REF] Markantonatou | PMWE conventions for examples containing multiword expressions[END_REF]. POS and morphological features use UD. We use the IETF BCP-47 standardized language codes in all examples.

Specific compositional semantic procedures are assigned to syntactic structures deemed regular[START_REF] Steedman | The Syntactic Process[END_REF].

The .cupt format instantiates the CONLL-U Plus metaformat meant for complementing UD with additional layers: https://universaldependencies.org/ext-format.html

Tokenization issues occur not only in compounds. Agglutinative languages may adopt different word segmentation strategies, in spite of similar structure[START_REF] Tae Hwan Oh | Analysis of the Penn Korean Universal Dependency Treebank (PKT-UD): Manual revision to build robust parsing model in Korean[END_REF]. This must also be addressed[START_REF] Tyers | Working Group 1 (What counts as a word?)[END_REF] but goes beyond this paper's scope.

14 Even if "idiomatic MWE" would be more

precise.15 One exception, in English, would be to abandon the semantic compound:prt vs. advmod distinction in VPCs.

Acknowledgements

This work started out as a discussion at the Dagstuhl Seminar 21351: Universals of Linguistic Idiosyncrasy in Multilingual Computational Linguistics. 16 We thank all the participants for inspiration. We are grateful to Schloss Dagstuhl, the Leibniz Center for Informatics, as well as the organizers of the event, for bringing us together.

We are also building upon the efforts of UniDive, the CA21167 COST Action: Universality, diversity and idiosyncrasy in language technology. 17 At the time of drafting this paper, UniDive was at the proposal stage and paved the way towards many of the ideas presented here.

Our work has also been partly funded by the French Agence Nationale pour la Recherche, through the SELEXINI project (ANR-21-CE23-0033-01).

We thank Jena Hwang for providing the Korean example (11).

//universaldependencies.org/ 2 https://gitlab.com/parseme/corpora/