Cooking Up Referring Expressions

COOKING UP REFERRING EXPRESSIONS Robert Dale Centre for Cognitive Science, University of Edinburgh 2 Buccleuch Place, Edinburgh EH8 9LW, Scotland email: rda~uk, ac. ed. epJ.stemi~nss, c s. ucl. ac. uk ABSTRACT This paper describes the referring expression generation mechanisms used in EPICURE, a com- puter program which produces natural language descriptions of cookery recipes. Major features of the system include: an underlying ontology which permits the representation of non-singular entities; a notion of diacriminatory power, to determine what properties should be used in a description; and a PATR-like unification grammar to produce surface linguistic strings. INTRODUCTION EPICURE (Dale 1989a, 1989b) is a natural lan- guage generation system whose principal concern is the generation of referring expressions which pick out complex entities in connected discourse. In particular, the system generates natural lan- guage descriptions of cookery recipes. Given a top level goal, the program first decomposes that goal recursively to produce a plan consisting of oper- ations at a level of detail commensurate with the assumed knowledge of the hearer. In order to de- scribe the resulting plan, EPICURE then models its execution, so that the processes which produce referring expressions always have access to a rep- resentation of the ingredients in the state they are in at the time of description. This paper describes that part of the system responsible for the generation of subsequent refer- ring expressions, i.e., references to entities which have already been mentioned in the discourse. The most notable features of the approach taken here are as follows: (a) the use of a sophisticated un- derlying ontology, to permit the representation of non-singular entities; (b) the use of two levels of se- mantic representation, in conjunction with a model of the discourse, to produce appropriate anaphoric referring expressions; (c) the use of a notion of dis- crimiaatory power, to determine what properties should be used in describing an entity; and (d) the use of a PATR-1ike unification grammar (see, for ex- ample, Karttunen (1986); Shieber (1986)) to pro- duce surface linguistic strings from input semantic structures. THE REPRESENTATION OF INGREDIENTS In most natural language systems, it is assumed that all the entities in the domain of discourse are singular individuals. In more complex domains, such as recipes, this simplification is of limited value, since a large proportion of the objects we find are masses or sets, such as those described by the noun phrases two ounces of salt and three pounds of carrots respectively. In order to permit the representation of enti- ties such as these, EPICURE makes use of a notion of a generalized physical object or physob]. This permits a consistent representation of entities irre- spective of whether they are viewed as individuals, masses or sets, by representing each as a knowledge base entity (KBE) with an appropriate structure at. tribute. The knowledge base entity corresponding to three pounds of carrots, for example, is that shown in figure 1. A knowledge base entity models a physobj in a particular state. An entity may change …

Content truncated to 500 words