Applying Lexical Rules Under Subsumption

Applying Lexical Rules Under Subsumption Erhard W. Hinrichs Dept. o[ Linguistics University of T~bingcn Wilhelmstr. ] ]3 D- 72074 Tiibingen Germany eh~sf s. nphil, uni-tuebingen, de Tsuneko Nakazawa NTT Laboratories 1-2356 Take, Yokosuka 238-03 ,Japan t suneko©nttnly, nit. j p Abstract Lexical rules are used in constraint- based grammar formalisms such as llead-Driven l)hrase Structure Grammar (IIPSG) (Pollard and Sag 1994) to ex- press generalizations atnong lexical en- tries. '['his paper discusses a number of lexical rules from recent I[PSG analy- ses of German (tlinri<;hs and Nakazawa 1994) and shows that the grammar in some cases vastly overgenerates and in other cases introduces massive spurious structural ambiguity, if lexical rules ap: ply under unification. Such l)rot)lems of overgeneration or spurious ambiguity do not arise, if a lexical rule al)plies to a given lexical ent;ry iff the lexical entry is subsumed by the left:hand side of the lexical rule. I,'inally, the paper discusses computational consequcnce~s of at)plying lexical rules under subsuml)tion. 1 Introduction Current linguistic theories place an increasing amount of grammatical information in the lexicon and employ a variety of mechanisms to express generalizations across lexical entries: templates (Flickinger 1987, Shieber 1986), inheritance hier- archies (Flickinger 1987, Pollard and Sag 11994), and lexical rules (Bresnan 1982, I)owty 1982, Gaz- daret al. 1985, Pollard and Sag 1994). Lexical rules (henceibrth: LRs) have been subjected to particularly close s<:rutiny. 'Fhis research has tb- cused on two important issues: 1. how the use of LRs affects the generative power of grammar formalisms and the computational complexity of parsing algorithms (Uszkoreit and Peters 1986, Carpenter 1991), and 2. how to provide a de- notational semantics for LRs (Calcagno and PoP lard 1995, Meurers 1995). In this paper we ad- dress neither of these two issues. Instead we will concentrate on a question that we c<msider to be of equal importance, but that has received sur- prisingly little attention: Under what conditions should an I,R be applicable to a given lexical entry (henceforth: LE)? For gramrnar formalisms that employ the notion of unitication of attribute-value structures, two criteria for applicability naturally suggest themselves: 1. llypolhcsis A: A lexical rule applies to a lexi- cal entry ifr the lexi<:al entry unifies with the left-hand side of t;he lexical rule. 2. Hypothesis B: A lexical rule applies to a lex- ical entry iff the lexical entry is subsumed by the left-hand side of the lexical rule) Without much argument, it is cotnmonly as- sume<t that llypothesis A is correct (el. Pollard and Sag 1994, (]alcagno and Pollar<t :1995, and Meurers 1995). This paper argues that 11ypoth- esis A should be rejected on empirical grounds. We discuss a number of Ll{s that have been used in IIPSG analyses of German (tlinri<:hs and Nakazawa 1994) and show that the grammar will either vastly overgenerate and accept ungrammat- ical sentences or introduce spurious structural am- biguity for grammatical sentences, if ltypothesis A is adopted. Itowever, no such problems ofovergeu- eration or spurious ambiguity arise, if one adopts llypothesis B, instead. …

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