«Are you Afraid of Ghosts?» A Proposal for Busting Predicate Ellipsis in U niversal D ependencies (2025)

Abstract: This paper addresses the representation of ellipsis in dependency syntax, proposing both a theoretical and a practical workflow for its analysis and annotation in treebanks, following the state-of-the-art Universal Dependencies framework. We discuss the challenges of annotating ellipsis, with a focus on predicate ellipsis and its representation in dependency treebanks, and emphasize the importance of accounting for such phenomena for syntactic analysis and machine learning applications. We present a case study based on the Italian-Old treebank, demonstrating the applicability of the proposed workflows and invite the community to participate in this initiative with their own languages.

## introduction a widely acknowledged principle in science is that the manner in which we choose to represent reality significantly shapes the nature of the material we aim to structure and interpret (kuhn, 1962). this is particularly true in the analysis of ellipsis, a syntactic phenomenon that represents the omission of linguistic material in a sentence (merchant, 1999), where the choice of the model to represent and encode (missing) linguistic information has a crucial impact on both its study and interpretation. studying a phenomenon that represents, by its nature, an absence is a challenging task. merchant (merchant, 2018, p.25) draws a compelling comparison of ellipsis with a black hole, stating that "detecting and arguing for such "missing" structures is analogous to searching for and determining the properties of a black hole: one can tell it's there only by its effects on surrounding material". due to this inherently elusive nature, since ellipsis is, by definition, silent in the data, an additional challenge arises in representing it within syntactically annotated corpora (treebanks), which are designed, among other purposes, to support the representation and queryability of syntactic phenomena. in the present paper, we address the representation of ellipsis in dependency syntax by providing both a theoretical and a practical workflow for analyzing and representing it in treebanks, in accordance with the state-of-the-art dependency framework, universal dependencies (de marneffe et al., 2021). the paper is structured as follows: in section 2, we provide a definition of ellipsis and outline key concepts. section 3 offers an overview of ellipsis within syntactic theory, with particular attention on dependency frameworks. it also presents how ellipsis is addressed in various dependency treebanks, with a specific focus on universal dependencies in subsection 3.1. in section 4, we discuss the importance of annotating ellipsis. sections 5 and 6 introduce the theoretical and practical workflows, respectively, along with the challenges they entail. finally, section 7 presents a case-study based on the italian-old treebank, and section 8 concludes the paper. ellipsis has been defined as an asymmetry between meaning and form in an expression (van craenenbroeck and temmerman, 2018). in this section, we will address some key concepts about ellipsis, in order to provide terminology and insights of the phenomenon. when speaking of ellipsis, we should consider the following aspects: in example 1, we provide an instance of ellipsis, highlighting all the aspects shown above: example 1 "i wish all happy holidays, and moreso, ___ peace on earth." @xcite the ellipsis site is marked with the "___". the remnants are "and", "moreso", "peace on earth". the antecedent that lead as to solve the ellipsis is composed by the subject "i", the verb "wish", the beneficiary "all", and, in this case, the elided material respects the identity condition, being a copy of the antecedent. @xcite therefore, the sentence with the elided material expressed will be as follows: ## ellipsis in syntax and in treebanks from a theoretical syntactic perspective, studies on ellipsis have been conducted mainly within constituency frameworks (ross, 1969; merchant, 2001; kennedy, 2003), which significantly outnumber those grounded in dependency syntax. constituency-based analysis of ellipsis tends to provide a broad classification of ellipsis types, foot_4 primarily grounded in the notion of constituent movement within the syntactic structure. however, within the dependency framework, ellipsis has not been extensively analyzed, positioning it as a relatively underexplored syntactic phenomenon. among the main studies on ellipsis from a dependency perspective, osbourne (osborne, 2019) offers a key contribution, namely the identification of the catena, a syntactic unit different from the constituent. his analysis and classification of ellipsis build and provide justification of licensing different type of ellipsis. ellipsis has also been defined as "unrealized words" by hudson (hudson, 2010), referring to covert words that lack pronunciation or spelling, with their unique distinction from overt ones being their inaudibility. regarding the representation of ellipsis in treebanks, approaches vary depending on the formalism adopted. in constituency-based treebanks, such as the penn treebank (ptb) (marcus et al., 1993) and the bultreebank (osenova and simov, 2003), which follows the head-driven phrase structure grammar (hpsg) formalism (pollard and sag, 1994), ellipses are explicitly annotated through the use of empty nodes. conversely, in dependency treebanks, the representation of ellipsis poses a greater challenge. this is primarily due to the principle that, in dependencybased annotation, the number of nodes in the tree corresponds exactly to the number of tokens in the sentence, thereby precluding the use of empty nodes. for instance, the prague dependency treebank (pdt) (hajič et al., 2017) addresses ellipsis explicitly through a dedicated attribute and requires its reconstruction at other annotation layers. @xcite in the following subsections 3.1 and 3.2, we address in detail the formalism adopted by the state-of-the-art dependency framework, namely universal dependencies. ## why do we need ellipsis? as mentioned in section 3.1, only a small amount of enhanced treebanks is available. providing an enhanced version is not mandatory in ud, and in most cases, the effort required to develop and maintain a basic treebank is already substantial enough to relegate the manual creation of an enhanced layer to a lower priority. additionally, the task of automati-cally enhancing a basic ud treebank is particularly challenging, especially when it comes to generating empty nodes to account for ellipsis (simi et al., 2018; droganova et al., 2018). moreover, attempts to automatically parse ellipsis in dependency treebanks using large language models (llms) have not yielded satisfactory results ( ćavar et al., 2024a,b), which has led to the development of corpora specifically focused on ellipsis. @xcite while it is clear that, at the current state of the art, a fully automatic reconstruction of ellipsis is not yet feasible, it is still possible to support the human annotation process by designing workflows that facilitate the manual annotation of ellipsis. before describing the procedures that assist manual annotation, in section 5, we will briefly outline the general steps that are essential for dealing with ellipsis. specifically, in this exploratory study, we focus on predicate ellipsis. @xcite ## "ghostbusting" ellipsis: a theoretical approach before moving on to a practical proposal for how to address ellipsis in ud (see section 6), we will first outline the general workflow required when dealing with ellipsis, namely with something that is absent from the data. we divide this workflow into three steps: detection, identification, and reconstruction. it is important to clarify that the proposed workflow -along with the order in which the steps are presented -is intended as a methodological proposal for handling ellipsis in annotated data. in other words, it does not address how ellipsis is processed in the mind of the reader or speaker -particularly with respect to the reconstruction phase -which is the subject of specific psycholinguistic studies (hofmeister, 2007), but lies beyond the scope of the present work. each step is described in detail below: however, each of these steps raises questions and presents challenges that deserve to be addressed and discussed. we will list them in their respective order, presenting the problems in forms of questions: the first challenge in the detection process involves the position of the ellipsis site. determining the position of the ellipsis site can become problematic, especially when languages with a relatively free word order are concerned. we will discuss a possible solution to this issue in section 7. an antecedent within the text is a challenging task (especially for a machine), identifying it from world knowledge is even more difficult, raising the question of whether and how it is possible to circumscribe it. since the aim is to make ellipsis explicit in order to analyze and recognize it, it is evident that reconstruction plays a crucial role in the task. however, it also presents significant challenges. the main risk is creating a cemetery of empty nodes, where the reconstruction of missing elements is exceedingly arbitrary. therefore, it is essential to carefully consider the extent of reconstruction of the empty node, both in terms of how much we should reconstruct (i.e., where the antecedent ends) and what information about the reconstructed node should be reported in the empty node. we will suggest possible solutions to some of these issues in the following sections. ## "ghostbusting" ellipsis: a practical workflow to deal with ellipsis in (e)ud building upon the considerations in section 5, we provide in this section a practical workflow for addressing ellipsis in (e)ud. this section is structured as follows: subsection 6.1 outlines a method for querying ellipsis in basic treebanks, while subsection 6.2 presents a new proposal for annotating ellipsis in eud. ## a case-study: predicate ellipsis in italian-old treebank. some preliminary results and considerations in this section, we present examples of the enhancement of predicate ellipsis in a portion of the italian-old treebank, following the method described in section 6. italian-old foot_21 is a native ud treebank containing the divine comedy by dante alighieri, an old italian poem written between approximately 1306 and 1321. the poem is divided into three cantiche: inferno (hell), purgatorio (purgatory), and paradiso (heaven). the first cantica, inferno, was manually annotated from scratch with respect to syntax. in contrast, the other two cantiche were pre-parsed using a model trained on inferno data and subsequently manually corrected (corbetta et al., 2023). the enhancement discussed here focuses on the first cantica, inferno, which comprises 33,416 tokens (excluding punctuation marks). in the following subsections, we first describe the extraction of predicate ellipsis (subsection 7.1), and then (subsection 7.2) report on some noteworthy cases of enhancement in light of the proposals presented in subsection 6.2. ## conclusion and future work in this paper, we focus on a specific syntactic phenomenon, ellipsis, that has been analyzed across various frameworks, though to a lesser extent within the dependency-based tradition. the inherent difficulty of capturing the nature of an omission in the text is compounded by the challenges of representing it graphically in syntactic corpora, namely treebanks. within the state-of-theart dependency framework (ud), ellipsis appears to be only marginally addressed, largely due to the annotation choices made at the basic level and the complexity involved in automatically retrieving such structures. to address the lack of gold-annotated data, which are crucial for both linguistic analysis and machine learning, in this work we experimented on predicate ellipsis, as a preliminary step towards the ultimate goal of developing language-independent guidelines for the treatment of ellipsis in universal dependencies. given the complexity of the topic, in this paper we have narrowed the scope to predicate ellipsis, with the aim of extending the analysis and annotation to all types of ellipsis in future works. for this paper, we focus on two complementary workflows: a theoretical one, centered on the analysis of ellipsis, and a practical one, aimed at providing a comprehensive annotation of predicate ellipsis in eud. examples of reconstructions, along with practical annotation cases, are presented in the final section with respect to the ud treebank italian-old. an enhanced annotation of predicate ellipsis will be provided in the next release of the treebank italian-old. given the inherently non-lexicalized nature of ellipsis, the proposed workflow can be extended to other languages through the use of morpho-syntactic annotation. concerning future work, a data-driven classification of predicate ellipsis in italian-old treebank is envisaged, one that emerges directly from the data and provides examples of ellipsis in context. additionally, we plan to develop a rule-based script, designed to be as language-independent as possible, to support the semi-automatic enhancement of predicate ellipsis. this step does not aim at a fully automatic resolution of ellipsis, which is still far from happening, but rather at facilitating manual annotation, which remains necessary. we welcome and encourage contributions from other treebank maintainers-or from researchers interested in exploring ellipsis-who wish to enrich their treebanks with this type of information.