Lexical Rules: What Are They? 1.1 Horizontal and Vertical Redundancy

Horizontal redundancy is inherent to lex-ica consisting of descriptions of fully formed objects. This causes an unwelcome expansion of the lexical database and increases parsing time. To eliminate it, direct relations between descriptions of fully formed objects are often deened. These are additional to the (Typed Multiple) Inheritance Network which already structures the lexicon. Many implementations of horizontal relations, however, fail to generate lexical entries on a needs-driven basis, so eliminate neither the problem of lexicon expansion nor that of ineecient parsing. Alternatively, we propose that lexical entries are descriptions of objects open to contextual speciica-tion of their properties on the basis of constraints deened within the type system. This guarantees that only those grammatical lexical entries are infered that are needed for eecient parsing. The proposal is extremely modest, making use of only basic inference power and ex-pressivity. 1 Lexical Rules: what are they? Within the strongly lexical framework of HPSG, lexical rules are used to express relations among descriptions|-a kind of indirect \horizontal re-latedness" (Pollard & Sag 1987, 209) which can be contrasted with the vertical relations between the type(s) of lexical elements. Type relations are, of course, captured directly as the monotonic (typed) multiple inheritance network itself, which structures the lexicon. Typical examples of horizontal redundancy in the hierarchical lexicon thus conceived are the Alternation phenomena (e.g. Dative Shift, the Locat-ive Alternation, etc) and word formation phenomena (innectional and derivational morphology). In fact, Pollard and Sag also refer to declension class membership and similar facts as horizontal relations , and as we shall see, the boundary between vertical and horizontal relations is not immutably xed once and for all. The notion of lexical rule is often given some status at the level of linguistic or psychological theory. (Pollard & Sag 1987) make reference to a generative or procedural interpretation of lexical rules as a deductive mechanism which can be deployed on a needs only basis, for example, to generate words from a single base form. The conception of lexical rules as essential generative devices (rather than static statements expressing (sub-) regularities), is shared in much innuential work (e.g. (Bresnan 1982), (Pinker 1989)), although it is by no means universal, even within HPSG. Viewed from an implementational perspective, on-they application of lexical rules brings with it a number of distinct advantages which follow from the drastic reduction in the size of the lexical database (lexical construction is less …