tailieunhanh - Báo cáo khoa học: "User-Defined Nonmonotonicity in Unification-Based Formalisms"

A common feature of recent unificationbased g r a m m a r formalisms is that they give the user the ability to define his own structures. However, this possibility is mostly limited and does not include nonmonotonic operations. In this paper we show how nonmonotonic operations can also be user-defined by applying default logic (Reiter, 1980) and generalizing previous results on nonmonotonic sorts (Young and Rounds, 1993). | User-Defined Nonmonotonicity in Unification-Based Formalisms Lena strõmbãck Department of Computer and Information Science Linkoping University S-58185 Linkõping Sweden Abstract A common feature of recent unificationbased grammar formalisms is that they give the user the ability to define his own structures. However this possibility is mostly limited and does not include nonmonotonic operations. In this paper we show how nonmonotonic operations can also be user-defined by applying default logic Reiter 1980 and generalizing previous results on nonmonotonic sorts Young and Rounds 1993 . 1 Background Most of the more recent unification-based formalisms such as TFS Emele and Zajac 1990 UD Johnson and Rosner 1989 CUF Dorre and Eisele 1991 and FLUF Stromback 1994 provide some possibility for the user to define constructions of his own. This possibility can be more or less powerful in different formalisms. There are however several constructions proposed as desirable extensions to unification grammars that cannot be defined in a general and well-defined way in these formalisms. One such class of constructions is those that have some degree of nonmonotonic behaviour. Examples of such constructions are any-values default-values and some constructions . constraining equations completeness and coherence used in LFG Kaplan and Bresnan 1983 . This paper describes a method that permits the user to define such nonmonotonic constructions. This is done through generalizing the work on nonmonotonic sorts Young and Rounds 1993 . This generalization results in a default logic similar to Reiter 1980 but where subsumption and unification are used instead of logical truth and consistency. There are three main advantages to Young and Rounds work compared with other approaches to default unification Bouma 1990 Bouma 1992 Russel et al. 1992 which justify choosing it as a starting point for this work. The first is the separation of definite and default information where .