Extending the Enhanced U niversal D ependencies – addressing subjects in pro-drop languages (2025)

Abstract: Enhanced Universal Dependencies (EUD) serve as a crucial link between syntax and semantics. Beyond basic syntactic dependencies, EUD provides valuable refined logical connections for downstream tasks such as semantic role labeling, coreference resolution, information extraction, and question answering. The original EUD framework defines six types of relationships, but this paper introduces an extension designed to address subject propagation in pro-drop languages. This “Extended EUD” proposal increases the number of relationships that may be annotated in sentences, improving linguistic representation. Additionally, we report our experiments on a corpus of Portuguese (a pro-drop language), which we make publicly available to the research community.

## introduction syntax-based approaches for multilingual natural language processing (nlp) have evolved in the last decade mostly due to the growing number of languages that have gold standard treebanks annotated under the universal dependencies framework @xcite . currently, there are 296 corpora available on the ud website, but only 42 of them foot_0 provide enhanced universal dependencies (eud) in the ninth column of the well-known conll-u format, called deps. eud constitutes a "bridge" between syntax and semantics. in column "deps", basic dependencies are transformed into a more semantic-like representation which associates each token to every head token they modify, regardless of whether this relation is explicit or implicit. for this reason, each token in eud may have multiple heads. besides that, eud includes case markers (dependents of cc, case and mark relations) in the relation name of their heads @xcite , providing helpful clues to their respective semantic roles. by making explicit logical relations that were previously implicit, eud increases the number of responses that can be obtained from an annotated corpus and paves the way for downstream applications that need these responses, such as semantic role labeling, coreference resolution, information extraction and question answering. eud allows for six types of "enhancements", including the propagation of subjects shared by coordinate clauses (conj clauses) and the assignment of "external subjects" for clauses with null subject (xcomp clauses). once a clause with implicit subject receives an enhanced subject, it can share it with clauses dependent on it, creating a propagation chain. this recursive nature of eud shows that one eud relation may rely on the product of another eud relation, and not only on basic syntactic dependencies. as an illustration, figure 1 shows a sentence with eud annotation where a nsubj is first propagated to a conj dependent clause and then to a xcomp dependent clause (in red edges). considering this recursive property of subject propagation within a sentence, eud could further leverage syntax, also including propagation of subject for acl, acl:relcl, advcl and ccomp. this would be particularly relevant for so-called pro-drop languages, which present high occurrence of subject ellipsis since a verb form encodes person, number and sometimes gender features that indicate who the subject is. if there is at least one explicit subject within a series of clauses in a sentence, such subject can be propagated to the other clauses with implicit subject. as it is possible to have different sequences of clauses in a sentence, the subject propagation path must be clear so as not to interrupt propagation. if, for example, a ccomp has an xcomp or conj as dependents, these clauses will only receive subject propagation once ccomp has received subject propagation before them. this may be observed if we compare figures 2 and 3 . whereas figure 2 follows the current eud guidelines and has only one subject, figure 3 , following the proposed extended eud, has four subjects, as ccomp needs to receive subject propagation from its head before propagating it to the xcomp and conj dependent clauses. it is important to note that ellipsis of the subject is not exclusive to pro-drop languages and other languages could take advantage of this extension in eud. in english, for example, it is not uncommon to find an advcl with subject ellipsis (in bold in the following examples): following his discharge, he taught english at a buddhist girls' school while also taking classes at kyoto university. 2 the meal was extremely overpriced and lacked flavor, especially for being a special nye menu. 3 however, when there is no explicit subject in a sentence, there is no possible propagation. in these cases, one might consider inserting an empty token to represent the elided subject, but this is a separate task, not related to subject propagation. moreover, eud emphasizes that empty tokens are only allowed for elided predicate insertion (predicates that are suppressed in the dependent clause but may be recovered from the matrix clause). we took on the task of implementing eud in portuguese and exploring the full propagation of explicit subjects for dependent clauses that have elided subjects, writing rules based on features typical of verb forms in pro-drop languages. the strategy for doing this was to customize a multilingual rule-based eud system, pre-annotate the corpus and manually revise the results, making changes to the rules, adding new rules, or fixing the ud gold standard annotation when necessary. this paper reports the finalized enhanced dependency annotation for the porttinari-base corpus @xcite . a detailed account of the annotation process is not included here, as it has already been partially described by @xcite . more emphasis will be placed on describing the annotation process of the proposed eud extension. the contributions of this paper are: the remainder of this paper is organized as follows: section 2 briefly presents the related work and section 3 reports the methodology we adopted; the results are reported in section 4 while section 5 explores the relation between basic dependencies and eud; some limitations and final remarks are presented in sections 6 and 7, respectively. ## related work one of the precursors of ud, the stanford dependency model, already included an output containing inferred relations @xcite , intended to provide a semantically richer representation of syntactic relations, making them more readily employable by other applications. the idea was to replicate modifiers for each token they modify and to replicate case markers for each token they introduce. the term "enhanced" first appeared in de @xcite referring to a series of transformations: marking external subjects and the external role in relative clauses, renaming dependencies to include figure 3: eud annotation extended to ccomp in "ele disse que quer viajar e já economizou para isso." (lit.: he said that wants to travel and has already saved up for it.) recognizing the relevance of such ideas, schuster and manning (2016) developed an english converter to produce an enhanced output according to ud restrictions, that is, expanding basic dependencies without changing them. nivre et al. (2018) evaluated two cross-lingual techniques to automatically annotate eud in ud treebanks: a rule-based english system and a datadriven finnish system. they used swedish and italian corpora to test both systems. the rule-based system performed better at assigning subjects, and the data-driven system performed better at propagating coordinate dependents. overall, both systems contributed to bootstrap the eud annotation. the proposal to develop non-languagedependent systems to annotate eud inspired two shared tasks held at iwpt: one in 2020 @xcite and another in 2021 @xcite . however, their multilingual dataset did not include portuguese. euds were instantiated in portuguese by @xcite and, later, @xcite developed a portuguese customization: the rule-based system udtoeud foot_4 @xcite , a graph rewriting system that uses grew @xcite to convert basic dependencies into eud. @xcite clarify that eud annotation is optional in treebanks, and it is allowed to annotate only one, several or all of the six types of eud. the ud framework, until now, does not allow the annotation of other types of eud out of the six types already described in the guidelines foot_5 . ## methodology revising eud annotation is an extremely heavy and time-consuming task, which requires even more qualified human resources than those needed to revise basic dependencies. to bootstrap eud annotation, we customized the udtoeud converter to portuguese (de @xcite , after a indepth linguistic study on enhanced dependencies for the portuguese language @xcite . we chose udtoeud because it has all the right qualities for our task: 1) it is based on rules, which enabled us to improve the system to increase the quality of the whole annotation without relying on previously annotated data; 2) it scored above 98% over gold basic ud data for italian, which is the language closest to portuguese among those that took part in the shared task; and 3) it allows entering language-specific lexicon: control and raise verbs (to determine the subject of xcomp), and adverbs (to determine which adverbs left to the head of conj should propagate to the conj dependent). by customizing udtoeud, we automatically annotated the six eud types in porttinari-base, which has 8,418 sentences. as the converter reached 96.05% elas in a gold standard eud composed of sentences assembled from porttinari-base, it was not deemed necessary to perform human-revision on a one-by-one basis in this phase. in the next phase, we improved the converter to include propagation of subjects to other clauses: ccomp, advcl, acl and acl:relcl. to find candidates for propagation, we used the following rules: besides that, for ccomp and advcl, their head should be head of nsubj or nsubj:pass, i.e., they should have a subject to propagate, regardless of whether it was an explicit subject or a subject resulting from a previous propagation. in other words, if the head does not have a subject to propagate, no propagation is possible. for acl:relcl and some types of acl, the head of the head should have a subject to propagate. another restriction to propagation was that the dependent clause (or its aux or aux:pass or cop) should have the same values for the features person and number of the clause (or its aux or aux:pass or cop) from which it would inherit the subject. as this is a new task, all the cases of propagation (2,147 sentences) and non-propagation (759 sentences) were verified by linguists that are experts in ud, leading to the improvement of the system. in this phase, we detected some non-recurrent errors in basic dependencies, which were promptly corrected. since we used a strategy of converting basic dependency trees into eud, eud automatic annotation only works if the basic dependencies are correctly annotated; hence they are a very good source for checking the basic dependencies logic. ## results in this section, we discuss separately the implementation of eud and eeud, but present the increase in relations achieved after each of these phases. ## eud as feedback on basic dependencies annotation when using a conversion strategy, eud and eeud strongly rely on the annotation in other conll-u columns, mainly on head (dependency head) and deprel (dependency relation), but also on upos (universal part-of-speech) and feat (column that encodes morphological features). in portuguese, several verbs have ambiguous forms in the first and third person, mainly in the past imperfect tense. as this feature is used to propagate subjects, annotation errors can lead to eud errors. when we noticed this type of error, we looked for all the verb forms that allowed more than one value for the person feature and revised their annotation. as already mentioned, we also revised the voice feature of past participles without a dependent aux:pass, as they may require @xmath0. when two predicates have different voice values, the respective relation of the propagated subject needs to be adjusted from nsubj to aux:pass (or the inverse), as shown in figure 6 . ## limitations the rules we implement rely heavily on comparing the person and number features of the head and the dependent: if they are the same, there is propagation; otherwise, there is not. however, even when these features have the same values, propagation may not work. this problem occurs mainly with @xmath1, as @xmath2. two examples illustrate the above problem. in the first example (figure 7 ), "o veículo" (the vehicle) is clearly not the subject of "acordou" (woke up), even though the propagation is licensed because the person and number rules have been met. in the second example (figure 8 ), only world knowledge allows us to detect the error, as the subject of "preferia" (would prefer) is the obl "exbailarina" and not "chefe" (boss), which is nsubj of the head. ## final remarks we produced two versions of the corpus: one with eud as described in the ud guidelines (corpus available on the ud website) and another with all enhanced relations, including those extensions we implemented, i.e., eeud foot_7 . working with a rule-based system to annotate enhanced dependencies has given us a deeper understanding of our language. at the same time, when we checked the automatically assigned enhancements, we noticed some inconsistencies in the annotation of the basic dependencies, which were corrected, improving the corpus quality as a whole. eeuds are dependency relations more difficult to annotate automatically, as the number of errors produced by the rules was greater than the number of errors in the original eud, and required many manual corrections. even so, the task proved to be productive, as we managed to increase the number of propagated subjects by 56%. drawing on our two versions of the golden standard corpus (one annotated with eud and another with eud + eeud), as future work, we intend to train automatic eud and eeud parsers for portuguese. we also plan to take a closer look at the enhancement of the orphan dependency relation. for this, we intend to produce data augmentation of orphan cases, since the sparsity of the data, added to the diversity of patterns, will probably prevent automatic approaches from achieving good performance in eud annotation. the rules of the customized version of ud-toeud are available at github foot_8 . we believe the rules we designed to deal with elided subjects may be useful for other languages, especially for the so-called pro-drop languages. extending eud to allow any kind of subject propagation would partially solve the inequality in the number of subjects between pro-drop and non-pro-drop languages, remaining cases where there is no subject to propagate. the gold standard corpus may also be useful for those wishing to experiment with a non-rulebased approach. figure 8 : error analysis -the logical subject of the ccomp should be the obl of its head (ex-bailarina): "o chefe perguntou à ex-bailarina se não preferia uma vaga em marketing" (lit.: the boss asked the former dancer if wouldn't prefer a job in marketing)