Revisiting and Amending Central Kurdish Data on UniMorph 4.0
Year: 2023

Abstract:
UniMorph–the Universal Morphology project is a collaborative initiative to create and main­ tain morphological data and organize numer­ ous related tasks for various language process­ ing communities. The morphological data is provided by linguists for over 160 languages in the latest version of UniMorph 4.0. This pa­ per sheds light on the Central Kurdish data on UniMorph 4.0 by analyzing the existing data, its fallacies, and systematic morphological er­ rors. It also presents an approach to creating more reliable morphological data by consider­ ing various specific phenomena in Central Kur­ dish that have not been addressed previously, such as Izafe and several enclitics.

heading: 1 Introduction
text: Computational morphology, the study of word for mation using computational methods, is one of the important tasks in natural language process ing (NLP) and computational linguistics. This field has been one of the prevailing and longstand ing tasks with many applications in syntactic pars ing, lemmatization andmachine translation (Roark and Sproat, 2007). There have been remarkable advances and paradigm shifts in approaches to analyze and generate morphology: starting from adhoc approaches in the earlier systems, then rule formalisms and finitestate models since the 1980s (Karttunen and Beesley, 2005) with the no table example of KIMMO twolevel morpholog ical analyzer (Karttunen et al., 1983), followed by statistical and classical machine learning since the 1990s as in (Goldsmith, 2001; Schone and Jurafsky, 2001), and more recently, approaches relying on neural network models since 2000s. Lastly, more robust techniques are proposed us ing monolingual data hallucination (Anastasopou los and Neubig, 2019), transfer learning (Kann et al., 2017) and pretrainedmodels (Hofmann et al., 2020).
Unlike the progress in approaches, the depen dence of systems on clean and reliable data, re gardless of the size, for accurate morphological analysis and generation has not changed much. In order to bring together various linguistic com munities to create datasets and incentivize fur ther studies in the field, the UniMorph1 (Batsuren et al., 2022) project has been a leading initia tive in this vein. In UniMorph 4.0, the latest version of the project, there are 168 languages from various language families for which mor phological data is provided according to the Uni Morph schema (SylakGlassman, 2016). Addition ally, the ACL Special Interest Group on Compu tational Morphology and Phonology (SIGMOR PHON)2 has played an important role to organize workshops and shared tasks using the UniMorph data. Some of the previous shared tasks focus on crosslinguality and context in morphology (Mc Carthy et al., 2019), unsupervised morphological paradigm clustering (Wiemerslage et al., 2021) and morphological inflection generation, segmen tation, and interlinear glossing in this year’s task. One of the languages that is of interest in this paper and is also included in UniMorph is Central Kurdish, also known as Sorani (ckb). Central Kur dish, as a variant of the IndoEuropean language Kurdish, has a fusional morphology with several distinctive features due to its splitergativity, er ratic patterns in morphotactics and, several endo clitics used in verbal forms. These characteristics seem to be known to the UniMorph community, as described in Pimentel et al. (2021, p. 8). However, the current data available for Central Kurdish con tains systematic errors and lacks coverage in mor phological forms. The data is also provided in a script that is not used by Kurdish speakers, thus of no utility to downstream tasks in reality. Con sequently, these result in poor performance of sys
1https://UniMorph.github.io 2https://sigmorphon.github.io
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tems that rely on the data in real scenarios.
Contributions This paper summarizes some of the salient features in Central Kurdish morphology. It also aims to discuss the main issues of Central Kurdish data on UniMorph 4.0. Moreover, the pa per provides a new dataset of quality with consid erable coverage and carries out experiments on the newly annotated data.

heading: 2 Central Kurdish Morphology
text: Kurdish is an IndoEuropean language spoken by over 25 million speakers in the Kurdish regions in Turkey, Iran, Iraq and Syria, and also by the Kur dish diaspora around the world (McCarus, 2007). Central Kurdish, also known as Sorani is the Kur dish variant that is mostly spoken by the Kurds within the Iranian and Iraqi regions of Kurdistan. Central Kurdish is a nullsubject language and has a subjectobjectverb (SOV) order and can be dis tinguished from other IndoIranian languages by its ergativeabsolutive alignment which appears in past tenses of transitive verbs (Ahmadi and Ma soud, 2020). In this section, we provide a brief de scription of Central Kurdish morphology by focus ing on morphemes and morphological processes.

heading: 2.1 Bound Morphemes
text: Morphemes are classified into free and bound. While free morphemes are meaningful as they are, bound morphemes only carry meaning when af fixed with other words. Bound morphemes are classified into two categories of affixes and cli tics. Affixes and clitics are similar in the way that they cannot constitute a word and they lean
on a prosodic host, i.e. a word for stress assign ment. Clitics can appear with hosts of various syn tactic categories while affixes only combine with syntacticallyrelated stems (Haspelmath and Sims, 2013, p. 198). The clitics and affixes in Central Kurdish have been widely studied previously and have been shown to be challenging considering the general theory of clitics (W. Smith, 2014; Gharib and Pye, 2018). This problem is particularly ob servedwith respect to the direct and oblique person markers which can appear in different positions within a wordform depending on the functional ity. In this section, the clitics and affixes in Central Kurdish are described. Figure 1 provides the most frequent clitics and affixes in Central Kurdish.

heading: 2.1.1 Clitics
text: Clitics are categorized based on their position with respect to the host. A clitic is called proclitic and enclitic, if it appears before and after the host, re spectively. There are two other forms of clitics which are nonperipherical and exist only among a few natural languages. If a clitic appears between the host and another affix, it is called a mesoclitic. A different type of nonperipheral clitic is endo clitic which appears within the host itself and is unique to a few languages around the world, such as Udi (W. Smith, 2014), Degema (Kari, 2002) and also Central Kurdish. Central Kurdish has two types of endoclitics: pronominal makers, also introduced as mobile per son markers by Walther (2012), and the emphasis endocliticشی =îş which can be translated as ‘also’ or ‘too’ (Ahmadi et al., 2023). The pronominal endoclitics function as agent markers for transitive
verbs in the past tenses or endoclitics as a patient marker for transitive verbs in the present tenses. This is due to the split ergativity feature of Central Kurdish where the agent and patient markers are specified differently. The following examples show the alignment in present and past tenses of نتوەک (KEWTIN, ‘to fall’) and نترگ (GIRTIN, ‘to get’). The agent marker ن in in intransitive present tenses (examples 1 & 3) serves as a patient marker in transitive past tenses (examples 2 & 4) due to ergativity while another morpheme نای =yan appears as the agent marker in the transitive verb (example 4).
(1) dekewin dekewin نوەکەد fall.PRS.PROG.INTR.3PL ‘(they) are falling.’
(2) degirin degirin نرگەد get.PRS.PROG.TR.3PL ‘(they) are getting.’
(3) kewtin kewtin نتوەک fall.PST.PROG.INTR.3PL ‘(they) fell.’
(4) girtyanin girt=yanin ننایترگ get.PST.PROG.TR.ERG.3PL.3PL ‘(they) got (them).’
Furthermore, the two endoclitic categories of Central Kurdish appear in an erratic pattern within a word form or a phrase. If a prefix appears be fore the stem of a transitive verb in the past tense, the agent marker postpends to the leftmost mor pheme; in other cases, the agent marker appears
after the verb stem following a varying morphotac tic rule depending on the tense, mood, aspect and transitivity of the verb. Table 1 presents an exam ple where the 1SG marker م (=im) and the empha sis endoclitic =îş appear after and before the host, i.e. ترگ (girt), depending on the presence of other bound morphemes such as negation prefix ەن (ne) or the verbal particle اد (da). It is worth mention ing that this pattern may vary based on the Central Kurdish subdialects. Moreover, the present form of the copula in Cen tral Kurdish are also used as clitics with nouns and adjectives. Table 1 shows these as enclitics.

heading: 2.1.2 Affixes
text: In comparison to clitics, a higher number of bound morphemes in Central Kurdish belong to affixes. Affixes can be categorized into inflectional and derivational based on their ability to create new lexemes. The most frequent affixes in Central Kur dish appear as prefixes and suffixes. Some of the inflectional affixes of Central Kurdish belonging to openclass parts of speech, namely nouns, verbs, adjectives and adverbs, are shown in Table 2. In ad dition, Izafe particle î and its allomorph e which appear between a head and its dependents in a noun phrase are frequently used to create possessive con structions, as in نم یوان (nawî min ‘my name’). In addition to compoundwords, Central Kurdish relies on derivationalmorphemes to create new lex emes, particularly new verbal lexemes. To this end, verbal suffix ەوە (ewe) and verbal particles such as اد (da) and ڵەه (heł) are used. It is worth not ing that the passive form of verbs is derived from the verb stem by using ێر/ار (ra/rê) or their allo morphs ێرد/ارد (dira/dirê) suffixes, unlike Kur manji Kurdish which relies on periphrastic forms with HATIN (to come) (Ahmadi, 2021b).
In the following, we summarize some of the dis tinct features of affixes in Central Kurdish.
Discontinuous Morphemes A morpheme that gets interrupted by the insertion of another morphological unit is known as a discontinuous morpheme. Two categories of discontinuous morphemes exist in Central Kurdish: a) demon stratives “em ...e” ‘this.DEM’ and ‘‘ew ...e” ‘that.DEM’ and, b) circumpositions such as “be ...da”, “le ...da” and “bo ...ewe”, respec tively meaning ‘through’, ‘in’ and ‘toward’ where “...” refers to the position of another mor phological unit between the two discontinuous morphemes. While the Latinbased orthography of Kurdish suggests writing such morphemes detached from the preceding word, they are usually concatenated in the PersoArabicbased script.
Postverbal Complement e and Pronominal Ad verb ê In Central Kurdish, a verb that has the valency of a prepositional phrase with prepositions be ‘to’ or bo ‘for’ can take the postverbal comple ment e to replace the preposition. In this case, it is compulsory for a noun phrase to come af ter the verb (Edmonds, 1961, p. 236). Further more, the pronominal adverb ê can replace the antecedent prepositional object, and the postverbal complement e, oblique pronoun, accusative nouns or locative adverb. This is particularly used with two verbs of DAN ‘to give’ and GEYIŞTIN ‘to arrive’. A more detailed description of Central Kurdish morphology, including adpositions and pronouns as freemorphemes, is provided in (Ahmadi, 2021a) and (Naserzade et al., 2023).

heading: 3 Central Kurdish on UniMorph 4.0
text: In this section, we analyze the existing morpholog ical data for Central Kurdish on UniMorph 4.0 and describe some of the current fallacies. The UniMorph project provides a dataset for Central Kurdish that contains 24,316wordforms.3
3Available at https://github.com/UniMorph/ckb
This dataset was initially created within the Alex ina Framework (Sagot, 2010) by Walther and Sagot (2010) and focuses on inflectional morphol ogy by providing a set of forms of the paradigms of 252 lemmas with noun or verb partofspeech tags. Overall, 33 morphological features based on Uni Morph are used in the dataset, including LGSPEC1 and LGSPEC2 which are respectively used for Izafe morpheme î and its allomorph e. The number of features combined together is 226 features for all the word forms. In other terms, 0.98% of the word forms are assigned a unique combination of features. Analogous to the notion of ‘leakage’ in syntactic parsing (Krasner et al., 2022) that reveals the overlap of the train and test sets, such a repeti tive usage of the features can cause an erroneously high performance of analysis models. As such, we believe that the dataset has very limited coverage of word forms and lacks diversity. In the following, we categorize some of the major issues of the Central Kurdish data on Uni Morph. Table 3 provides a few examples based on the dataset and categorizes their issues as well.

heading: 3.1 Unconventional Writing
text: Unlike Northern Kurdish which is mostly written in a Latinbased Kurdified script known as Be dirxan’s orthography, Central Kurdish is more con ventionally written in a PersoArabic script. The Kurdish data on UniMorph is written in an un conventional Latinbased orthography that is not used in practice. Furthermore, the character <i> for phoneme /I/ is not represented in the selected script, even though it is frequently used in many morphemes and undergoes various morphophono logical alternations. This phoneme, also known as Bizroke (Ahmadi, 2019), is represented by <i> in the Latinbased script of Kurdish while is miss ing in the PersoArabic script. We transliterate the original forms in the dataset in the Latinbased script of Kurdish in Table 3.

heading: 3.2 Morphotactics
text: As described in § 2, Central Kurdish has a com plex morphotactics when it comes to verbs. This is also reflected in the inflection of verbal forms of the UniMorph dataset where some verbal word forms do not conform to themorphology of Central Kurdish and its dialects. This is particularly ob served in transitive verbs in which the agent mark ers should appear before the verb stem and after the leftmost prefix in past tenses (see §2.1.1). How ever, this morphotactic rule is not systematically present in the verb forms. It is worth mentioning that this phenomenon is not the case in closely related variants, i.e. Northern Kurdish and South ern Kurdish, or the closelyrelated language Per sian. Therefore, we believe that the annotation was mistakenly and inaccurately carried out under the influence of such variants and languages.

heading: 3.3 Morphophonological Alternations
text: Many morphemes in Central Kurdish alter based on morphophonological rules. This is particu larly the case of bound morphemes starting with a vowel, such as eke as the singular definite marker and e as a demonstrative suffix that respectively appear as ke/yeke and ye depending on the pre ceding phoneme. In the UniMorph data, such alter nations are not consistently taken into account. An eyecatching issue of this type is N;FOC which is as sociated with nouns that appear with the clitic =îş. The allomorph =ş of this clitic that appears after vowels seems to be universally used in the dataset
regardless of the morphophonological rule. There fore, word forms associated to this tag and other similar tags like N;LGSPEC2 are potentially wrong.

heading: 3.4 Incorrect Morphemes
text: A less severe problem of incorrect inflections is due to incorrect morphemes, particularly al lomorphs. We believe that the unconventional script may have aggravated such issues. For in stance, the singular imperative form of verbs, i.e. v;imp;sg are missing the suffix e as in the in correct form of bbexš (bibexş) instead of bibexşe (FORGIVE.IMP.2SG) and the morpheme yě (yê) is frequently and incorrectly used instead of the mor pheme ye to indicate the conditional mood of the verb. Nevertheless, such issues have been dis cussed, particularly concerning allomorphs, within the UniMorph community (Gorman et al., 2019). Taking these issues into account, we estimate that 25% of the forms of Central Kurdish data on UniMorph 4.0 are incorrect.

heading: 4 Methodology
text: Given the fallacies of the Central Kurdish dataset on UniMorph 4.0, we believe that a new dataset is required for a thorough morphological analysis of this language. Although we correct the exist ing dataset on UniMorph 4.0, we also extend it with new lemmata and more complete paradigms. This measure was taken to ensure the quality of the forms based on a corpus and more importantly, in both conventional scripts of Kurdish, namely the
PersoArabicbased and the Latinbased scripts. In this section, we discuss our approach to creating a new dataset for Central Kurdish.

heading: 4.1 Modeling Central Kurdish on UniMorph
text: During the data preparation process, we noticed that the UniMorph schema described by Sylak Glassman (2016) lacks several features that are commonly used in not only Central Kurdish but also, most Iranic languages, such as Izafe (Wind fuhr, 2009). In the schema, the label LGSPEC with a consistent ID is considered for languagespecific features. Using this, we also introduce a few fea tures that are currently unsupported and map these new features to LGSPEC with an ID to be consistent with the current schema of UniMorph. Table 4 pro vides a list of such features.
It is worth noting that in the current Central Kur dish data on UniMorph 4.0, LGSPEC1 and LGSPEC2 are respectively used for Izafa suffix <î/y> and its allomorph <e>. Similarly, the endoclitic =îş is specified as FOC. These are the only language specific tags that are currently used in this dataset.

heading: 4.2 FiniteState Transducers
text: Relying on Naserzade et al. (2023)’s finite state transducers, we develop a morphological analyzer and generator that can handle all possible well formed inflected forms of a given word in Central Kurdish. The analyzer takes a word and yields all possible morphological tags. Similarly, the gener ator takes as input a lemma and its partofspeech tag, in addition to the past and present stems and transitivity for verbs, and inflects the lemma ac cordingly. The output words are formed according
to Central Kurdish standard orthography and mor phophonological rules. The number of forms with unique features is 3,032 for a general noun lemma, 9,096 for a gradable adjective, 3,180 for a transi tive verb, and 636 for an intransitive verb. Figure 2 illustrates a transducer to generate noun forms.

heading: 4.3 Data Generation
text: Using the finitestate transducers, we generate two datasets containing a diverse set of word forms and partofspeech tags as follows:
Goldstandard We first randomly extract 1,000 words from Veisi et al. (2020)’s corpus and then use the finitestate transducers to analyze them. Given that the transducers do not take word con text into account, this step was followed by a man ual verification to make sure that only the relevant analysis and tags are selected based on the context.
Silverstandard We also create another dataset that contains full paradigms for 10 nouns, 5 ad jectives, 17 intransitive verbs (including three pas sive and two causative verbs) and 12 transitive verbs. As this dataset doesn’t rely on context, we refer to it as silverstandard. These words are listed in Table A. To cover all morphophonological changes that occur in the inflectional forms, we se lect words having stems ending with a consonant, vowels <a, e, ê, î, o, û>, approximants <y> and <w/>, and diphthong <wê>. Note that vowels <i> and <u> do not occur in wordfinal positions. During the generation processing, we set a few restrictions in our dataset. In the conjugation of transitive verbs, it is not possible to have both the subject and object pronouns in either the first or second person. This is due to the reflexive con
struction in Central Kurdish that does not com monly appear in the verb form. For example, *de m=nas=im ‘*I knowme’ and *detannasît ‘*you know you’ are illformed. For this purpose, the ad verb xo ‘self’ is commonly used.
Table 5 summarizes the number of forms in our datasets in comparison to the current UniMorph 4.0 data. We present our datasets in both con ventional scripts of Kurdish, the Arabicbased and Latinbased ones. The latter is more widely used for Northern Kurdish facilitating crossdialectal comparisons. Moreover, we provide the corpus based context of word forms and our customized tags in Table 4 in a separate dataset.

heading: 5 Analysis

heading: 5.1 Morphological Reinflection
text: To evaluate our datasets, we carry out an analy sis on morphological reinflection introduced as the nonneural baseline for task 1 of SIGMORPHON 2018 that extracts lemmatoform transformations heuristically (Cotterell et al., 2018). To do so, we first shuffled the datasets and created a 70–10–20 train–dev–test split. During the process, we made sure that identical samples were selected in the two scripts to make the comparison of performances valid. We then run the nonneural baseline using
the train sets of the three datasets and evaluate the models on the three test sets. Table 6 presents the accuracy and average edit distance in the three datasets. Although it would have been interesting to compare the performance of the baseline system across test sets, e.g., training and testing on differ ent datasets, such comparison could only be valid if the same set of tags has been used which is not the case in the current UniMorph data. Based on the results of the systems that participated in the SIGMORPHON 2021 Shared Task on morpholog ical reflection (Pimentel et al., 2021), an accuracy of over 90% can be achieved.

heading: 5.2 Error Analysis
text: In order to better understand the challenges of rein flection models, we manually checked the wrong outputs of the models trained and tested on our data to determine failure points. Since we have generated all possible inflectional forms of sev eral lemmas and the data is shuffled before build ing the model, some complex forms do not oc cur in the train set. Therefore, the model failed to cover those forms. Another difficulty of the baseline model is in tackling the morphophonolog ical changes. As we have covered stems with dif ferent final phoneme types, the majority of errors that have lower edit distance are in handling these changes. For example, the failure in alternating the indefinite suffix ‘êk’ to ‘yek’ after a vowel is a primary source of the errors. In Kurdish, verbs have different past and present stems. For many verbs, the present stem is made by removing the final consonant or vowel of the past stem; for instance, the past and present stems of girtin ‘to get’ are girt and gir, respectively. However, numerous exceptions enforce computa tional studies to consider the present verb stems as irregular and look them up from a table, as in the present stems łê or bêj for gutin ‘to say’ and xo for xwardin ‘to eat’. Analyzing the reinflectional er
rors showed that detecting such alternations is an other major source of error. Regarding the accuracy based on the scripts, the accuracy of the baseline on data written in the Latinbased (L) script is slightly lower than the Arabicbased script (A). This can be explained by the missing character Bizroke (i) in the Arabic based script that plays an important role in Central Kurdish morphology (see §3.1) while the Latin based character uses it.

heading: 5.3 Inflectional Synthesis Degree
text: As an additional analysis, we calculate the synthe sis degree of inflected forms in Central Kurdish by averaging the number of morphemes per form in the gold and silver datasets. According to the de grees reported by Greenberg (1960), Central Kur dish has a relatively high synthetic degree of 2.22 comparable to Old English (2.12), Yakut (2.17), and Swahili (2.55). Among the selected partof speech tags, adjectives exhibit the highest level of synthesis as they can function with nominal affixes and clitics and also, a few other distinct ones such as tir and tirîn as comparative and superlative suf fixes. Although prefixing is not used in nouns and ad jectives of Central Kurdish, verbs have a higher synthesis in prefixing, mainly due to the verbal pre fixes related to negation such as ne, na and me but also subjunctive bi and progressive markers e and de. Moreover, transitive verbs show the highest ratios of synthesis in prefixing in compari son to intransitive verbs. This is due to the erratic patterns of pronominal endoclitics that may appear before or after the stem, while that’s not the case in intransitive verbs (see §2.1.1). It should be noted that these results are expected to be different in derivational morphology.

heading: 6 Conclusion
text: In this paper, we discuss some of the fallacies of the current data of Central Kurdish on UniMorph 4.0. We argue that the dataset is not only lacking coverage but also misrepresents Kurdish morphol ogy by incorrect morphemes, unconventional writ ing and inaccurate morphotactics. Additionally, we propose a new dataset with a few additional la bels for some of the features of Central Kurdish, such as Izafe and various clitics. Our dataset is gen erated using finitestate transducers with the hu man in the loop and are transliterated in the Latin based script of Kurdish in addition to the Perso Arabicbased ones. The transliteration of the word forms facilitates comparative studies, particularly with Northern Kurdish which is mainly written in a Latinbased script. For each wordform, we also look it up in a corpus and provide the context in addition to the morphological features. Moreover, we create a baseline by training models in various setups and evaluating them on our dataset and the current Central Kurdish data on UniMorph 4.0. Fi nally, we suggest this dataset be added to the future version of UniMorph.
Limitations One of the limitations of our dataset is the lower number of wordforms belonging to a closeclass partofspeech as we chiefly focus on nouns, verbs (transitive and intransitive) and ad jectives. On the other hand, we only include in flectional morphology without paradigms of word formation. Furthermore, we only address the mor phology of the standard variety of Central Kurdish, i.e. that of Sulaymaniyah. We plan to extend our work to include other varieties of Central Kurdish along with derivational morphology. Given that Central Kurdish lacks a treebank, it will be com pelling to bridge Central Kurdish morphology and syntax as well. Another limitation of the current work is due to the UniMorph schema. Using the LGSPEC tag is not recommended for features that are found across languages but for those that are limited to specific languages (SylakGlassman, 2016, p.30). Given that some of the features of Central Kurdish, such as Izafe and pronominal copula, are also found in other closelyrelated languages, we believe that the current schema should be extended to use specific tags for such features or a better schema, akin to Guriel et al. (2022)’s hierarchical model, is needed for languages with rich morphology like Kurdish.

heading: Acknowledgments
text: The authors are grateful for the constructive com ments of the anonymous reviewers. Sina Ahmadi is generously supported by the National Science Foundation under DLIDEL award BCS2109578.

heading: A Appendix

heading: Future Work
text: Fi nally, we suggest this dataset be added to the future version of UniMorph. Limitations One of the limitations of our dataset is the lower number of wordforms belonging to a closeclass partofspeech as we chiefly focus on nouns, verbs (transitive and intransitive) and ad jectives. On the other hand, we only include in flectional morphology without paradigms of word formation. Furthermore, we only address the mor phology of the standard variety of Central Kurdish, i. e. that of Sulaymaniyah. We plan to extend our work to include other varieties of Central Kurdish along with derivational morphology. Given that Central Kurdish lacks a treebank, it will be com pelling to bridge Central Kurdish morphology and syntax as well. Another limitation of the current work is due to the UniMorph schema. Using the LGSPEC tag is not recommended for features that are found across languages but for those that are limited to specific languages (SylakGlassman, 2016, p. 30). Given that some of the features of Central Kurdish, such as Izafe and pronominal copula, are also found in other closelyrelated languages, we believe that the current schema should be extended to use specific tags for such features or a better schema, akin to Guriel et al. (2022)’s hierarchical model, is needed for languages with rich morphology like Kurdish