Overcoming the challenges in morphological annotation of T urkish in universal dependencies framework (2021)

Abstract: This paper presents several challenges faced when annotating Turkish treebanks in accordance with the Universal Dependencies (UD) guidelines and proposes solutions to address them. Most of these challenges stem from the lack of adequate support in the UD framework to accurately represent null morphemes and complex derivations, which results in a significant loss of information for Turkish. This loss negatively impacts the tools that are developed based on these treebanks. We raised and discussed these issues within the community on the official UD portal. This paper presents these issues and our proposals to more accurately represent morphosyntactic information for Turkish while adhering to guidelines of UD. This work aims to contribute to the representation of Turkish and other agglutinative languages in UD-based treebanks, which in turn aids to develop more accurately annotated datasets for such languages.

## introduction universal dependencies 1 (henceforth, ud) is an international cooperative project for annotating treebanks with a standardized format to facilitate the development of automated support for natural language processing @xcite . at present (version 2.8) it includes 202 treebanks from 114 different languages. in this paper, we outline and propose solutions to two main challenges for the ud based 1 the official website is https://universaldependencies.org/ . the first challenge that we address is the lack of representation of derivations in ud, which causes annotators to take derived forms of nouns, verbs, adjectives, and adverbs as lemmas. turkish has a morphology that uses very rich and productive derivational suffixes. @xcite states that a turkish word may have up to six derivational suffixes at the same time. without any tool for derivation, for example, a word like okuldakilerden (from the ones in school) would have the lemma okuldaki, losing the annotation for the morphemes okul (school), and -da (locative case suffix) that comes before the derivation. since the english translation of the same word consists of five different words, from the ones in school, it is represented with five dependencies in the ud framework. it annotates much more information than the current annotation scheme of turkish does. this unparallelism may harm future cross-linguistic analyses. properly locating the roots of such derived nouns in the system might be also useful in lemmatization processes. in this paper, we aim to contribute to the representation of derivation in ud, which will be helpful for the annotations of agglutinative languages like turkish. the second challenge is the lack of official rep-resentation for null morphemes. accounting for this type of morphemes is also considered vital by our team, since the turkish copula is frequently encountered as a null morpheme. except for present tense copula, most turkic treebanks split copulas which alleviates the problem. however, we did not find any account for the present tense copula which is always null. here, we also propose an annotation for the turkish present tense copular paradigm. besides these two challenges, we also target the presentation of a more complete annotation scheme for the commonly used yet ambiguous turkish verb ol-(to be), for which no clear annotation scheme exists in the turkish ud treebanks such as boun @xcite and imst-ud @xcite . we detected that different usages of olwere usually being used interchangeably with each other. in this paper, we introduce a list of all the oltypes and propose an annotation scheme for each. according to our proposed solutions for the outlined challenges, we started re-annotating the recently introduced boun treebank (2,000 sentences have been annotated so far), which is one of the largest ud-based turkish treebanks @xcite . our re-annotation of boun treebank entails all of the morphological and syntactic annotations in the ud framework. that is, we add, remove or change lemmas, parts-of-speech (pos) tags, features and their values, dependency relations and other miscellaneous information as per the decisions taken by the annotation team. our team is comprised of three linguists and four computer scientists who are all native speakers of turkish. in the morphological part of the 2,000 sentences that we have annotated, we changed 5,418 parts-ofspeech tags, added 12,654 features, and removed 9,513 features. in the syntactic part, we segmented 701 words into multiple words as well as altered 5,456 dependency relations and 7,333 heads. we also annotated 2,895 slots in the miscellaneous information tab. while taking decisions for the annotation scheme, the main objective is to represent the turkish morphosyntax as accurately as possible in accordance with the ud guidelines. 2 we believe, the proposed annotation solutions will contribute to more precise representation not only for turkish, but also for other agglutinative languages in the ud framework as well. the remainder of the paper is organized as follows. section 2 includes a brief survey about turkish treebanks. section 3 deals with the problem of derivation in ud. we touch on the difficulties of representing turkish in a derivation-free framework such as ud. in section 4, we comment on the representation of the null present tense copula in ud which discourages the representation of any type of null elements in a sentence. in section 5, we explain how we resolve the multilayered ambiguity problem the verb ol-(to be) -a polysemous multi-functional verb -creates for ud. section 6 concludes the paper. ## related work as stated in the ud website ud started as a joint result of stanford dependencies @xcite @xcite , google universal parts-of-speech tags @xcite and the interset interlingua for morphosyntactic tagsets @xcite . treebank efforts date back much earlier. with @xcite and @xcite , we see the earliest treebanks in english. about a decade later turkish treebanks started to emerge when @xcite and @xcite introduced the metu-sabancı treebank (mst) which consists of 5,635 sentences. this treebank was then revisited by @xcite to alleviate some issues attributed to the corpus like excessive parsing difficulty and cross-parser instability. as a result, they proposed the itu-metu-sabancı treebank (imst). in @xcite they converted imst into ud framework via a semi-automated process and mapping, naming it as imst-ud which is one of the first turkish ud treebanks. this treebank was later reannotated by @xcite , improving the annotation of embedded structures and core versus non-core dependents. the first turkish treebank that adopts the ud framework is grammar book treebank (gb) by @xcite . gb treebank draws 2,803 sentences or sentence fragments from the grammar book of @xcite and uses tr-morph @xcite to provide morphological annotation. one downside of this treebank is that it mainly consists of textbook examples rather than naturally occurring language. another important treebank effort was turkish-pud treebank which was published as a part of the conll 2017 shared task on multilingual parsing from raw text to universal dependencies @xcite . turkish-pud treebank consists of 1000 sentences which are first annotated as per the annotation guidelines of google and then converted into the ud framework. this treebank is especially noteworthy, since it has parallel annotations for 18 different languages, making it a useful resource for cross-linguistic analyses. this treebank was later re-annotated by @xcite to make it more linguistically consistent. one of the largest turkish ud based treebanks, boun treebank foot_1 , has been presented recently by @xcite with 9,761 manually annotated sentences. these sentences are a subset of turkish national corpus (tnc), a corpus first presented by @xcite that consists of more than 50 million words. the boun treebank sampled a great variety of text types, which are essays, broadsheet national newspapers, instructional texts, popular culture articles, and biographical texts. before the manual annotation, they used an end-to-end pipeline @xcite to parse raw texts to ud dependencies in conll-u format. the output was then re-annotated by turkish linguists. this is also the treebank our team is currently working on. ## problems in derivation one of the ud's main pitfalls when it comes to annotating a language like turkish is the lack of derivation in the system. being mostly a syntaxbased project, ud only prefers to mark syntactically relevant morphological distinctions, namely the inflectional morphology. the ud website gives the example of the word organizations in english whose lemma is marked as organization as opposed to organize. the suffix -tion is not annotated. in a way, the suffixes till the last derivation are ignored by the system. we can show the general idea as follows. say, infl1, infl2,... are inflectional suffixes and der1, der2,... are derivational suffixes. for a word like root + infl1 + der1 + infl2 + infl3 + der2 + infl4, the entirety of root + infl1 + der1 + infl2 + infl3 + der2 is a fixed, unprocessed, single lemma according to the ud guidelines. der2, the last derivative suffix here, blocks access to the annotation of the previous part (i.e. infl1 + der1 + infl2 + infl3), making all of them invisi-ble to the system -examples are provided in section 3.1 to illustrate this phenomenon. this is because derivation is not annotated and it is included as a part of the lemma under the ud framework. the current solutions for the languages that use derivation are not unified. for example, quechua, a language with an extremely agglutinative morphology @xcite , devised a derivation annotation strategy explained in a ud thread. 4 they use word split immensely and annotate verbs using expressions like root_vderiv_vderiv instead of features. they also add the functions of those derivations like vroot|+dir|+inch where +dir and +inch represent functions of each individual derivative suffix. since this method annotates each derivative suffix precisely, it provides a more fine-grained approach for derivation annotation than our approach. however, one downside for this approach is that it has a very distinct annotation style than those we see on other treebanks. a different system than classical feature and value system proposed by the ud guidelines is utilized in this treebank. one other derivation scheme we examined was that of finnish. in the finnish tdt treebank, they annotate derivations under the features tab using an additional feature called derivation. the value of this feature is the derivative suffix itself, e.g. @xmath0. this approach is useful since it marks the derivative suffix itself. however, with this scheme, it is hard to annotate a derived item when it has more than one derivative suffix. turkish sometimes uses derivation so productively, even to the point that it becomes syntactically significant. in the following, we will introduce the derivation related problems turkish presents and how we handled them via the tools permitted under the ud guidelines. ## lack of null morphemes in ud, there is no official way to represent null items. this presents a challenge for turkish since one of the allomorphs of turkish copula is a null morpheme. this section presents potential annotation problems in ud stemming from the lack of null morphemes, and proposes a solution without imposing radical changes such as introducing null morphemes to the ud framework. turkish copula has three allomorphs: -y-, iand ∅ @xcite @xcite . ionly surfaces when the copula is written separately. when it is connected to the predicate it can appear as -yor ∅ depending on the last sound of the predicate, respectively, being a vowel or a consonant. examples in (11) show the separated variation of the turkish copula for the past tense. examples in (12) show the same sentences but with the attached form of the copula. the only difference might be the slightly less formal register in the sentences in ( 12 ). the sentences in (11) and ( 12 ) have the identical meaning. the only difference is that in (12) copulas are in affix form. except for the present tense copula, all other affixed copulas are already segmented in previous turkish treebanks. we added segmentation of the present tense copula which is always null as shown in (13). the previous annotation of this sentence is shown in figure 3a . we consider this annotation problematic since the features slot contains only the features of the first person copular suffix -(y)im, that is @xmath1. in order to represent the features of the nominal predicate, like ögrenci (student), we decided to split the present tense copula as shown in figure 3b -not a common practice in turkic ud treebanks. 11 in turkish, the 3rd person paradigm almost always bears a null suffix. this means that we are unable to split it as there is nothing to split. ## conclusion having a highly rich morphology, turkish poses a great deal of challenges for linguists regarding representation in a dependency syntax framework. some of these challenges come from the rich morphology and some from the unexpected behavior of derivation in turkish. in this paper, we laid out some problems with respect to derivation and lexical ambiguities in the language and their adaptation to the ud framework. especially in the derivation part, we feel that some significant amount of work awaits us and other linguists who aim to enhance annotation of turkish in a syntactic framework based on dependency relations. we expect this study to contribute towards a more precise annotation scheme for turkish and other agglutinative languages, which in turn may also lead to the training of more accurate nlp tools including lemmatizers and dependency parsers.