A U niversal D ependencies Treebank for H ighland P uebla N ahuatl (2024)

Abstract: We present a Universal Dependencies (UD) treebank for Highland Puebla Nahuatl. The treebank is only the second such UD corpus for a Mexican language, and supplements an existing treebank for another Nahuatl variant. We describe the process of data collection, annotation decisions and interesting syntactic constructions, and discuss some similarities and differences between the Highland Puebla Nahuatl treebank and the existing Western Sierra Puebla Nahuatl treebank.

## introduction annotated linguistic corpora are an essential component of natural language processing (nlp), and are fundamental for training and evaluating models and applications. furthermore, consistentlyannotated corpora enable quantitative, comparative linguistic analyses. the universal dependencies (ud) project @xcite is a widely-used annotation framework whose aim is to provide a consistent schema for representing morphological and dependency-based syntactic phenomena for all of the world's languages. since a ud corpus contains rich information for all aspects of a standard nlp pipeline (tokenization, part-of-speech tagging, morphological analysis and syntactic parsing), it is well-suited as a resource for nlp application development. ud treebanks have also been successfully used in quantitative syntax research @xcite and large, multilingual typological linguistic studies @xcite . the development of annotated linguistic corpora for endangered, indigenous, and/or marginalized languages is critically important to ensure their inclusion in the domain of digital language technology as well as to improve their representation in, and therefore the validity and robustness of, multilingual corpus-based studies. in this paper, we present one such corpus, created using the ud framework, for highland puebla nahuatl, a language spoken in central mexico. this corpus will not only contribute to the linguistic study and nlp development for the highland puebla variant, but is also an important addition to the other existing nahuatl treebank (for the western sierra puebla variant) in that it can enable research into quantitative morphosyntactic dialectology and multi-dialectal nlp for nahuatl. highland puebla nahuatl, (or sierra puebla nahuatl, also referred to by inali as náhuatl del noreste central, iso-639-3 azz, henceforth hpn) is a nahuatl variant group spoken by about 70,000 people (ethnologue's 2007 estimate) in the northeastern sierra region of the state of puebla, mexico (see figure 1 ) in 24 municipalities @xcite . this particular nahuatl variant has been the subject of documentary and descriptive linguistic efforts @xcite @xcite , and there are at least two published dictionaries (key and richie de @xcite . intergenerational transmission of hpn is declining, resulting in language shift. along with all other indigenous languages in mexico, hpn is considered at risk of being lost @xcite . hpn is a member of the eastern branch of nahuatl (also commonly referred to as aztec) along with huasteca nahuatl, gulf nahuatl, and pipil/nawat. its speakers are believed to have brought the language to the sierra de puebla region either during the first migration of nahuatl-speakers from northwestern mexico @xcite , or as part of an early migration out of the valley of mexico in around 800 c.e. @xcite . in terms of isoglosses, hpn is a 't-dialect' since it has /t/ where central nahuatl varieties typically have /tl/, has /i/, instead of /e/, corresponding to proto-uto-aztecan **u, and has a number of distinguishing lexical features @xcite . ## related work despite ud's recent wide adoption in the computational linguistics community and over 100 ud treebanks for a diverse set of languages, treebanks for languages of the americas are few: corpora of over 10,000 tokens for mbyá guaraní @xcite , k'iche' @xcite , western sierra puebla nahuatl (wspn) @xcite , a slightly smaller (9,000 tokens) treebank for guajajara, and a number of very small corpora (less than 2,000 tokens) for various languages of brazil, e.g. apurina @xcite and st. lawrence island yupik @xcite . @xcite describe an earlystage, ongoing effort to annotate a treebank of classical nahuatl, the set of nahuatl varieties represented in texts from the early colonial period. work on resources and nlp development for hpn includes a large audio corpus with transcriptions and translations @xcite which has been used to train asr and speech translation models @xcite . @xcite present a finite-state morphological analyzer for the language, developed on a superset of the data used for the present treebank. research on the syntax of contemporary nahuatl includes flores nájera (2019)'s treatment of the simple clause of the nahuatl spoken in tlaxcala, and explorations of nonconfigurationality and polysynthesis in hueyapan nahuatl @xcite and western sierra puebla nahuatl @xcite . additional research in this area has focused on specific syntactic constructions such as relative clauses @xcite @xcite and anti-passives @xcite . aditionally, a non-trivial amount has been written about classical nahuatl morphosyntax @xcite @xcite , and the impact of contact on language change in nahuatl over time @xcite . we see the development of this and other morphosyntactically-annotated corpora for nahuatl as a significant contribution to this area. ## corpus the treebank data come from four sources: ## annotation decisions automatic processing was used to put the source text into conll-u format. the lemma, part-ofspeech, and morphological analysis were all initially performed automatically using a finite-state morphological analyzer for the language. the second author manually reviewed the automated results, and annotated the syntactic trees for all of the sentences using the ud annotatrix annotation tool @xcite . then, the first author reviewed all of the trees and any disagreements were discussed, with the differing views argued for, until both came to an agreement. the trees were updated with arborator grew @xcite . this approach to annotation was chosen because, at the moment, there is no well-defined set of guidelines for nahuatl ud parsing. thus the creation of this treebank, in conjunction with prior work on wspn, serves to further define and document annotation guidelines. in the following sections, we describe in more detail the processes and decisions made during annotation, highlighting a few noteworthy constructions in the treebank. ## comparing two nahuatl treebanks in this section, we take stock of the two existing nahuatl treebanks, of the highland puebla (hpn) (the present paper) and western sierra puebla (wspn) @xcite variants, comparing some basic linguistic properties and metadata in order to better inform future work that uses these corpora for comparative analysis or nlp. genre: the two nahuatl treebanks have quite different genre profiles: the hpn treebank presented here breaks down into essentially half spoken monologue/dialogue discussing plants and plant uses, and the other half non-fiction written text about the same topic. the wspn treebank is more diverse with respect to its genre distribution, with substantial portions of the text containing fiction, spoken personal narratives, and elicited example sentences. number of tokens, types, and trees: the two treebanks have nearly the same number of tokens, 10,356 and 10,088, respectively, though the hpn treebank achieves this number of tokens with far more trees (about 300 more), with a shorter average sentence length. this is expected given the nature of the data sources for each treebank described above. figure 2 shows the difference in sentence length distribution between the treebanks. the type-token ratios of the two treebanks, 0.23 for wspn and 0.18 for hpn, also suggests greater lexical diversity in the wspn treebank. ## training a highland puebla nahuatl parser in addition to corpus-based linguistic analyses, one of the principal uses of an anntotated treebank is the for the development of an nlp pipeline. to accompany the above discussion of the linguistic resource itself, here we present the results of training a neural model on the treebank data. we use the machamp toolkit (van der goot et al., 2021) to fine-tune contextual embeddings from the pretrained multilingual bert (mbert) model foot_9 on lemmatization (lemma), part-of-speech tagging, morphological analysis, and dependency parsing. the model leverages multi-task learning, such that all of the tasks share encoder parameters, but each has its own unique decoder: a transformation-rule classifier @xcite for lemmatization, a softmax layer on the contextual embeddings for part-of-speech tagging and morphological analysis, and a deep biaffine parser for dependency parsing @xcite . for all tasks we use the default hyperparameters. we trained the model for 100 epochs, selecting for each fold the epoch with the best performance on the evaluation set, where performance was defined as the sum of the accuracies of each task. thus, we could potentially achieve better results by selecting the best performing model for each task independently. since the goal in the present the discrepancy in the frequency of aux is, as discussed in the text, due to the wspn variant's use of the antecessive auxiliary o-, a dialectal feature not present in hpn. the differences in advmod and det frequencies can be attributed to a combination of dialectal difference, genre, and potentially annotation inconsistencies. the results, shown in the "in domain" column of ## concluding remarks we have presented a ud treebank for highland puebla nahuatl, which accompanies a treebank for a different nahuatl variety as one of two ud treebanks for indigenous languages of mexico (and, incidentally, one of only two ud treebanks for nahuatl languages). we observe a number of similarities with the existing nahuatl treebank, as well as some noteworthy novel syntactic constructions. additionally, we trained a neural model on the treebank, demonstrating the ability to build a relatively high-performing nlp pipeline for the language. we leave detailed experimentation of the nlp pipelines for nahuatl as future work. furthermore, we hope that this treebank in conjunction with the other nahuatl treebank enables quantitative exploration of syntax within the domain of nahuatl dialectology.