A finite-state morphological analyser for Highland Puebla Nahuatl
Year: 2023

Abstract:
This paper describes the development of a free/open-source finite-state morphological transducer for Highland Puebla Nahuatl, a Uto-Aztecan language spoken in the state of Puebla in Mexico.1 The finite-state toolkit used for the work is the Helsinki Finite-State Toolkit (HFST); we use the lexc formalism for modelling the morphotactics and twol formalism for modelling morphophonological alternations. An evaluation is presented which shows that the transducer has a reasonable coverage—around 90%—on freely-available corpora of the language, and high precision— over 95%—on a manually verified test set.

heading: 1 Introduction
text: This paper describes a new morphological analyser for Highland Puebla Nahuatl, an endangered language spoken in the state of Puebla in Mexico (see Figure 12). The analyser is based on finitestate technology, which means that it can be used for both the analysis and the generation of forms — a finite-state morphological transducer maps between surface forms and lexical forms (lemmas and morphosyntactic tags).
An analyser of this sort has a wide variety of uses, including for automating the process of corpus annotation for linguistic research as well as for creating proofing tools (such as spellcheckers) and for lemmatising for electronic dictionary lookup for language learners — in a language with heavy prefixing and suffixing morphology, determining the stem is not a simple matter.
Our approach is based on the Helsinki FiniteState Toolkit (HFST, Lindén et al. (2011)).
1https://github.com/apertium/apertium-azz 2Figure 1 is based on work by users TUBS (https://commons.wikimedia.org/wiki/File: Puebla_in_Mexico_(location_map_scheme).svg) and Battroid (https://commons.wikimedia.org/wiki/ File:Mexico_Puebla_Puebla_location_map.svg)

heading: 2 Prior art
text: Finite state transducers (FST) for modeling morphology has a long history within the field of computational linguistics (Kornai, 1996; Beesley and Karttunen, 2003).
Work on morphological analysers for Nahuatl languages includes an effort, inspired by literate programming, to use the code for the transducer as a descriptive grammar of a Nahuatl variety spoken in the state of Guerrero (Maxwell, 2015), and morphological analysers specifically targeting colonial-era Nahuatl, either for the exploration of colonial texts (Thouvenot, 2009), or as a means to evaluate similarity between written Nahuatl varieties (Farfan, 2019). One drawback of these projects is that they are not to our knowledge freelyavailable or easily-accessible.
Nicolai et al. (2020) describe the development of morphological analysers and generators for more than one thousand languages using the Johns Hopkins University Bible Corpus (McCarthy et al., 2020), including some variants of Nahuatl (however, not Highland Puebla Nahuatl).
Pugh et al. (2021) presents the first open-source morphological analyser for the Western Sierra Puebla Nahuatl variant group. Tona et al. (2023) expand on that system, extending it to support Huasteca Nahuatl. This latter work, however, has not been released.

heading: 3 Highland Puebla Nahuatl
text: Nahuatl (or Nahuat, Nahual) is a polysynthetic, agglutinating Uto-Aztecan language continuum spoken throughout Mexico and Mesoamerica. The Mexican Government’s Instituto Nacional de Lenguas Indígenas (INALI) recognizes 30 distinct variants (INALI, 2009).
Highland Puebla Nahuatl, (or Sierra Puebla Nahuatl, also referred to by INALI as Náhuatl del noreste central, ISO-639-3 azz) is a Nahuatl vari-
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ant group spoken in the Northeastern Sierra region of the state of Puebla, Mexico, mainly in the municipalities of Tetela de Ocampo, Zacapoaxtla, and Cuetzalan. According to Ethnologue’s 2007 estimate, it is spoken by an estimated 70,000 speakers.
This particular Nahuatl variant has been the subject of a number of descriptive works (Key, 1960; Robinson, 1970; Key and Key, 1953) and dictionaries (Key and Richie de Key, 1953; Cortez Ocotlán, 2017).

heading: 4 Data
text: The source data used to develop the FST comes from three sources: (1) A dataset of transcribed recordings of interviews and conversations, mainly about plants (Amith et al.), (2) a subset of texts in the azz variant from the multi-variant parallel corpus Axolotl (Gutierrez-Vasques et al., 2016), and (3) technical publications by the Sociedad Mexicana de Física3, which consist of translations of various scientific texts. The breakdown of volume for each of these sources is presented in Table 1.

heading: 5 Orthography
text: Writing practices in Nahuatl vary and are characterized by multiple competing views (de la Cruz Cruz, 2014). The most well-known and widelydisseminated orthographic standards for Nahuatl are ACK, a colonial-inspired orthography named after scholars Anderson, Campbell, and Karttunen, who popularized it in their work, the standard from the Instituto Nacional de Lenguas Indígenas (INALI) (INALI, 2018), and that used by the Secretaría de Educación Pública (SEP). In practice, Nahuatl writing contains a great deal of ortho-
3https://site.inali.gob.mx/SMF/Libros2.0/ nhtl/index.html
graphic variation, often even within the writing of a single author.
The orthography used for building the analyser follows what was taught in the Nahuatl course for adult learners given in the municipality of Tetela de Ocampo, Puebla in the summer of 2022 (TO). This broadly follows the SEP, but with the addition of the letter h which is used before u for /w/ after vowels or at the beginning of words. For example SEP ueueyi, TO huehueyi ‘big’, SEP mochiua, TO mochihua “it is made”.
We maintain a separate finite-state transducer to account for orthographic and spelling variation. This includes rules for orthographic changes like ts (SEP, INALI) → tz (ACK) (e.g. tejuatsin ‘youHON’ → tehhuatzin), spelling changes, such as w$ → j$ and abbreviations that are found in the transcriptions from the spoken corpora, such as ^t’ → ^tik.

heading: 6 Methodology
text: In this section, we outline some of the implementation details of the analyzer, including a description of relevant linguistic features.

heading: 6.1 Lexicon
text: The lexicon consists of around 5,000 lexemes which were added in frequency order (calculated using the corpora described in §4) and with reference to the two available dictionaries (Key and Richie de Key, 1953; Cortez Ocotlán, 2017) for part-of-speech classification. The lexicon was created in the lexc formalism, which is standard in HFST.
Closed categories (pronouns, conjunctions, etc.) were added manually based on class notes and on existing grammatical descriptions (Key, 1960; Robinson, 1970; Cortez Ocotlán, 2017).

heading: 6.2 Tagset
text: The tagset is based on the tagset of the Apertium project (Forcada et al., 2011), each tag is encased in greater than ‘<’ and less than ‘>’ symbols. The tag names are mnemonic, some of them coming from other analysers in the Apertium project and being based on English, Spanish, or Catalan terms, and some are based on Nahuatl terms. We include a conversion from this Apertium-based tagset to one based on Universal Dependencies (Nivre et al., 2020).

heading: 6.3 Morphotactics
text: The morphotactics of Highland Sierra Nahuatl is very similar to that of other Nahuatl varieties. It is characterised by a concatenative affixing morphology with a large number of inflectional and derivational morphemes. It also features long-distance dependencies between prefixes and suffixes.

heading: 6.3.1 Nouns
text: Nouns inflect for number and possession. They also have very productive derived forms, such as the reverential -tsin (1) and less productive derivations, such as -k(o) for locative, and can appear as predicates with the addition of subject prefixes. We implement the morphotactics for inflection and for the most frequent subset of the derived forms. Nouns are therefore split into separate continuation classes for their different combinatorial possibilities.
(1) kikouaj ki-koua-j O.SG3-buy-S.PL in in the
tokniuantsitsin to-kni-uan-tsi~tsin POSS.PL1-person-PL-PL.HON “People buy it.” (lit. “Our brethren buy it”)
In (1), the noun (i)kni ‘sibling’ appears with the first person plural possessive prefix to-, the
possessed plural marker -uan, and the reverential marker tsi~tsin, where plurality is further marked with partial reduplication of the -tsin morpheme.
Relational nouns: There is also a subcategory of nouns, called “relational nouns,” used for expressing spatial and temporal relations, as well as other non-core semantic roles. Unlike common nouns, these nouns have obligatory possession.
(2) In In mochiua mo-chiua O.REFL-make kuoujtaj, kuoujtaj, mountains, in in eua eua born
talixko, tal-ix-ko, ground-RELN-LOC, amo amo NEG itech i-tech POSS.SG3-on kuapalak. kuapalak. tree.trunks “It grows in the mountains, it comes up from the ground, it doesn’t grow in tree trunks.”
In (2) we see two methods in which relational nouns can be used. The first is talixkowhere the the relational noun -ixko ‘in front of / on the surface of’ is compounded with the noun tali ‘ground/earth’. This relational noun itself is composed of ix ‘face’ and ko a locative morpheme.
The second method is using a free-standing relational noun with a complement, itech kuapalak ‘in rotten tree trunks’, is composed of a possessive form of the relational noun -tech ‘on’ and the noun compliment kuapalak ‘tree trunk’.
These relational nouns can also appear separated from their complement, as in (3, where the complement of iuan ‘with’ is emol ‘beans’, but it appears to the right of the verbal complex se kikua “it is eaten”.
(3) uan uan and iuan i-uan POSS.SG3-with se se one kikua ki-kua O.SG3-eat emol emol beans
“... and it is eaten with beans”
They can also receive reverential morphology as in one of the typical ways of expressing goodbye, mohuantsin ‘with you’ (4).
(4) mohuantsin mo-huan-tsin POSS.2SG-with-HON “with you”
Locatives: In addition to compounding with relational nouns there is also a locative derivational suffix -k(o) which forms locative nouns from places. For example ima ‘her hand’, imako ‘in her hands’.4

heading: 6.3.2 Verbs
text: Verbs inflect for number and person of subject and object(s), and for tense, aspect and mood. They also can be compounded with auxiliary verbs and can have incorporated adverbial items for both direction of movement and for manner of action. Additionally there is reverential agreement for the second person.
(5) Xe Xe QST ma ma OPT nimitsonchiya ni-mits-on-chiya S.SG1-O.SG2-HON-wait huan huan and
tisentakuaskej? ti-sen-ta-kua-s-kej S.PL1-TOGETHER-O.NN3-eat-FUT-S.PL “Shall I wait for you and we’ll eat together?”
In (5) we see examples of incorporated adverbials, tisentakuaskej “wewill eat together”, affixal agreement, ti-[...]-kej for the first person plural subject and ta- for the indefinite object and the future tense suffix -s. The verb nimitsonchiya has the on- prefix, indicating reverentiality towards the addressee.
(6) se se one mokouilia mo-kou-ilia O.REF-buy-APP komo komo if se se one
kikuasneki. ki-kua-s-neki O.SG3-eat-FUT-want “One goes and buys it if one wants to eat it.”
4Although the name is the same, these locatives are unlike those found in other languages as inflection because: (1) not every word can take a locative suffix, (2) they are not selected for by argument structure, (3) the resulting meaning can be idiosyncratic. For this reason we categorise them as derivation as opposed to inflection.
(7) se se one kiualkui ki-ual-kui O.3SG-VEN-bring
“It is brought.” (lit. One brings it (here))

heading: 6.4 Morphophonology
text: Phonological processes are implemented via twol rules. There are relatively few of these, and they include degemination (/kk/ →[k]) and nasal assimilation (/n/ →[m] // m).

heading: 7 Results
text: To evaluate the analyzer, we calculate the naïve coverage for both tokens and types. The naïve coverage is reported for each data source in Table 1. Naïve coverage is the percentage of surface forms in a given corpus that receive at least one morphological analysis. Forms counted by this measure may have other analyses which are not delivered by the transducer.

heading: 7.1 Evaluation
text: Since we don’t have a large, annotated dataset for evaluation, we performed a manual inspection of two random samples of data to get a sense of the the system’s precision and to understand the reasons for any missed words.
First, we sampled 100 random analyses from the corpora and identified any mistakes. The precision on this sample was 95%. Next, in order to find out where the most work remains to be done with respect to coverage, we randomly sampled 100 types that are currently not recognised by the system. These words were categorised by part of speech, and in addition we marked each with one or more of the following seven error categories: (1)missing morphotactics, (2) missing orthographic normalisation, (3) missing compound word, (4) reduplication, (5) loan word / code-switching, (6) tokenisation error, and (7) missing lexicon entry.
Over half of all unknownwords were verb forms. Of these, five were caused bymissing orthographic normalisation rules, for example t’titipitstoti is an abbreviated form of tiktitipitstoti ‘you will be blowing the fire’, and 10 were due to missing stems in the lexicon.
Around ten percent of the sampled unknown words were caused by errors in tokenisation. The speech corpus contains false starts, for example amo nike..., amo nikmati “I don’t kn..., I don’t know”, and these do not currently receive any analysis.

heading: 8 Concluding remarks
text: We have described a robust finite-state morphological analyser for Highland Puebla Nahuatl. This work contributes to the recent increased focus in language technologies for Nahuatl, and may play an important role in supporting further Nahuatl language technology in the future.
In future work we would like to expand the lexicon to include more stems, to increase the coverage of all of the corpora, and to obtain new corpora for testing. We intend to include support for compounding and incorporation and for weighting the transducer. We already have 10,000 tokens manually disambiguated and will use these to weight more probable analyses.

heading: Acknowledgements
text: We would like to thank Patricia Aguilar Romero, don Pedro Rivera, and Mitsuya Sasaki for their help with the work described in this manuscript. In addition we would like to thank the anonymous reviewers for their helpful comments.

heading: Future Work
text: This work contributes to the recent increased focus in language technologies for Nahuatl, and may play an important role in supporting further Nahuatl language technology in the future. In future work we would like to expand the lexicon to include more stems, to increase the coverage of all of the corpora, and to obtain new corpora for testing. We intend to include support for compounding and incorporation and for weighting the transducer. We already have 10,000 tokens manually disambiguated and will use these to weight more probable analyses