Lexical and Syntactic Rules in a Tree Adjoining Grammar

LEXICAL AND SYNTACTIC RULES IN A TREE ADJOINING GRAMMAR Anne Abeill6* LADL and UFRL University of Paris 7-Jussieu abeille@zeta.ibp.fr ABSTRACT according to this definition 2. Each elementary tree is constrained to have at least one terminal at its frontier which serves as 'head' (or 'anchor'). Sentences of a Tag language are derived from the composition of an S-rooted initial tree with other elementary trees by two operations: substitution (the same operation used by context free grammars) or adjunction, which is more powerful. Taking examples from English and French idioms, this paper shows that not only constituent structures rules but also most syntactic rules (such as topicalization, wh-question, pronominalization ...) are subject to lexical constraints (on top of syntactic, and possibly semantic, ones). We show that such puzzling phenomena are naturally handled in a 'lexJcalized' formalism such as Tree Adjoining Grammar. The extended domain of locality of TAGs also allows one to 'lexicalize' syntactic rules while defining them at the level of constituent structures. 1 INTRODUCTION TO 'LEXICALIZED' GRAMMARS 1.1 Lexicalizing Phrase Structure rules In most current linguistic theories the information put in the lexicon has been increased in both amount and complexity. Viewing constituent structures as projected from the lexicon for example avoids the often noted redundancy between Phrase Structure rules and subcategorization frames. Lexical constraints on the well-formedness of linguistic outputs has also simplified the previous transformational machinery. Collapsing phrase-structure rules into the lexicon is the overt purpose of 'lexicali7ed' grammars as defmed by Schabes, Abeill6, Joshi 1988 : a 'lexicallzed' grammar consists of a fmite set of elementary structures, each of which is systematically associated with one (or more) lexical item serving as 'head'. These structures are combined with one another with one or more combining operation(s). These structures specify extended domains of locality (as compared to CFGs) over which lexical constraints can be stated. The 'grammar' consists of a lexicon where each lexical item is associated with a finite number of structures for which that item is the 'head'. We here assume familiarity with Tree Adjoining Grammars, which are naturally 'lexicalized' * The author wants to thank Yves Schabes, Aravind Joshi, Maurice Gro~, Sharon Cote and Tilman Becker for fruitful discussions, and Robert Giannasi and Beatrice Santorini for their help. Schabes, Abeill~, Jo~hl 1988 show that context free grammars cannot in general be lexicalized (using substitution only as the combining operation). They also show that lexicalized grammars are interesting from a computational point of view since lexicalization simplifies parsing techniques, because the parser uses only a relevant subset of the entire grammar: in a first stage, the parser selects a set of elementary structures associated with the lexical items in the input sentence, and in a second stage the sentence is parsed with respect to this set. As shown by Schabes, Joshi 1989, a parser's performances are thus improved. We show here that such 'lexicalization' should be extended to other components of the grammar as well, thus challenging the usual distinction between 'lexical' and 'syntactic' rules. Further …

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