Detecting Erroneous Uses of Complex Postpositions in an Agglutinative Language

This work presents the development of a system that detects incorrect uses of complex postpositions in Basque, an agglutinative language. Error detection in complex postpositions is interesting because: 1) the context of detection is limited to a few words; 2) it implies the interaction of multiple levels of linguistic processing (morphology, syntax and semantics). So, the system must deal with problems ranging from tokenization and ambiguity to syntactic agreement and examination of local contexts. The evaluation was performed in order to test both incorrect uses of postpositions and also false alarms. 1 Structure of complex postpositions Basque postpositions play a role similar to English prepositions, with the difference that they appear at the end of noun phrases or postpositional phrases. They are defined as “forms that represent grammatical relations among phrases appearing in a sentence” (Euskaltzaindia, 1994). There are two main types of postpositions in Basque: (1) a suffix appended to a lemma and, (2) a suffix followed by a lemma (main element) that can also be inflected. (1) etxe-tik house-(from the) from the house (2) etxe-aren gain-etik house-(of the) top-(from the) from the top of the house The last type of elements has been termed as complex postposition. We will use this term to name the whole sequence of two words involved, and not just to refer to the second element. Com© 2008. Licensed under the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported license (http://creativecommons.org/ licenses/by-nc-sa/3.0/). Some rights reserved. plex postpositions can be described as: (3) lemma1 + (suffix1 + lemma2 + suffix2) In these constructions, the second lemma is fixed for each postposition, while the first lemma allows for much more variation, ranging from every noun to some specific semantic classes. The above description (3) is intended to stress (with parentheses) the fact that the combination of both suffixes with the second lemma acts as a complex case-suffix that is “appended” to the first lemma. Both suffixes present different combinations of number and case, which can agree in several ways, depending on the lemma, case or contextual factors. Table 1 shows the different variants of two complex postpositions, derived from the lemmas bitarte and aurre. For example, the lemma bitarte is polysemous (“means, by means of, instrument, while (temporal), between”). Multiple factors affect the correctness of a postposition, including morphological and syntactic constraints. We also discovered a number of relevant contextual factors, which are not explicitly accounted for in standard grammars.