Structure-Sharing in Lexical Representation

Structure-Sharing in Lexical Representation Daniel Fllekinger, Carl Pollard, and Thomas Wasow Hewlett-Packard Laboratories 1501 Page Mill Road Palo Alto, CA.. 94~O3, USA Abstract The lexicon now plays a central role in our imple- mentation of a Head-driven Phrase Structure Grammar (HPSG), given the massive relocation into the lexicon of linguistic information that was carried by the phrase structure rules in the old GPSG system. HPSG's gram- max contains fewer tha4z twenty (very general) rules; its predecessor required over 350 to achieve roughly the same coverage. This simplification of the gram- max is made possible by an enrichment of the structure and content of lexical entries, using both inhcrit~nce mechanisms and lexical rules to represent thc linguis- tic information in a general and efficient form. We will argue that our mechanisms for structure-sharing not only provide the ability to express important linguistic generalizations about the lexicon, but also make possi- ble an efficient, readily modifiable implementation that we find quite adequate for continuing development of a large natural language system. 1. Introduction The project we refer to as HPSG is the current phase of an ongoing effort at Hewiett-Pa~',axd Labo- ratories to develop an English language understanding system which implements current work in theoretical linguistics. Incorporating innovations in the areas of lexicon, grammar, parser, and semantics, HPSG is the successor to the GPSG system reported on at the 1982 ACL meeting z Like the GPSG system, the current im- plementation is based on the linguistic theory known Generalized Phrase Structure Grammar, 2 though in- corporating insights from Carl Pollard's recent work on Head Grammars ~ which lead us to employ a richer lex- icon and a significantly smaller grammar. We report here on the structure of our lexicon, the mechanisms used in its representation, and the resulting sharp de- cre~e in the number of phrase structure rules needed. 4 I Gawron, et al. (1982). 2 Gazdax, Klein, Puilum, and Sag (1985). 3 Pollard (1984). 2. Mechanisms employed We employ three types of mechanisms for structure- sharing in our representation of the lexicon for the I-IPSG system: inheritance, lexical rules, and an Ol>- eration to create nouns from ordinary database enti- ties. In order to present a detailed description of these mechanisms, we offer a brief sketch of the representa- tion language in which the lexicon is constructed. This language is a descendant of FRL and is currently under development at HP Labs. s Those readers familiar with frame-based knowledge representation will not need the review provided in the next section. 2.1. The representation language The basic data structures of the representation lan- guage axe frames with slots, superficially analogous to Pascal records with fields. However, frames axe linked together by mew of class and inheritance links, such that when a particular frame F0 is an instance or sub- class of a more general frame F1, information stored in F1 can be considered part of the description of F0. For example, our lexicon database contains frames specify- ing properties of classes of …

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