Parsing preferences with Lexicalized Trey Adjoining Grammars exploiting the derivation tree

Parsing preferences with Lexicalized Tree Adjoining Grammars : exploiting the derivation tree Alexandra KINYON TALANA Universite Paris 7, case 7003, 2pl Jussieu 75005 Paris France Alexandra.Kinyon@linguist.jussieu.fr Abstract Since Kimball (73) parsing preference principles such as "Right association" (RA) and "Minimal attachment" (MA) are often formulated with respect to constituent trees. We present 3 preference principles based on "derivation trees" within the framework of LTAGs. We argue they remedy some shortcomings of the former approaches and account for widely accepted heuristics (e.g. argument/modifier, idioms...). Introduction The inherent characteristics of LTAGs (i.e. lexicalization, adjunction, an extended domain of locality and "mildly-context sensitive" power) makes it attractive to Natural Language Processing : LTAGs are parsable in polynomial time and allow an elegant and psycholinguistically plausible representation of natural language 1. Large coverage grammars were developed for English (Xtag group (95)) and French (Abeille (91)). Unfortunately, "large" grammars yield high ambiguity rates : Doran & al. (94) report 7.46 parses / sentence on a WSJ corpus of 18730 sentences using a wide coverage English grammar. Srinivas & al. (95) formulate domain independent heuristics to rank parses. But this approach is practical, English-oriented, not explicitly linked to psycholinguistic results, and does not fully exploit "derivation" i e.g. Frank (92) discusses the psycholinguistic relevance of adjunction for Children Language Acquisition, Joshi (90) discusses psycholinguistic results on crossed and serial dependencies. information. In this paper, we present 3 disambiguation principles which exploit derivation trees. 1, Brief presentation of LTAGs A LTAG consists of a finite set of elementary trees of finite depth. Each elementary tree must <<anchor>> one or more lexical item(s). The principal anchor is called daead>>, other anchors are called <<co-heads>>. All leaves in elementary trees are either <<anchor>>, <<foot node>> (noted *) or <<substitution node>> (noted $). These trees are of 2 types • auxiliary or initial 2. A tree has at most 1 foot-node, such a tree is an auxiliary tree. Trees that are not auxiliary are initial. Elementary trees combine with 2 operations : substitution and adjunetion. Substitution is compulsory and is used essentially for arguments (subject, verb and noun complements). It consists in replacing in a tree (elementary or not) a node marked for substitution with an initial tree that has a root of same category. Adjunction is optional (although it can be forbidden or made compulsory using specific constraints) and deals essentially with determiners, modifiers, auxiliaries, modals, raising verbs (e.g. seem). It consists in inserting in a tree in place of a node X an auxiliary tree with a root of same category. The descendants of X then become the descendants of the foot node of the auxiliary tree. Contrary to context-free rewriting rules, the history of derivation must be made explicit since the same derived tree can be obtained using different derivations. This is why parsing LTAGs yields a derivation tree, from 2 Traditionally initial trees are called o~, and auxiliary trees 13 585 which a derived tree (i.e. constituent tree) can be obtained. (Figure 1) 3 . Branches …

Content truncated to 500 words