TAILIEUCHUNG - Báo cáo khoa học: "A LOGICAL SEMANTICS FOR NONMONOTONIC SORTS"

Suppose we have a feature system, and we wish to add default values in a well-defined way. We might start with Kasper-Rounds logic, and use Reiter's example to form it into a default logic. Giving a node a default value would be equivalent to saying "if it is consistent for this node to have that value, then it does." Then we could use default theories to describe feature structures. The particular feature structure described would be the structure that supports the extension of the default theory. This is, in effect, what the theory of nonmonotonic sorts gives you. This. | A LOGICAL SEMANTICS FOR NONMONOTONIC SORTS Mark A. Young Bill Rounds Artificial Intelligence Laboratory The University of Michigan 1101 Beal Ave. Ann Arbor MI 48109 marky rounds@ Abstract Suppose we have a feature system and we wish to add default values in a well-defined way. We might start with Kasper-Rounds logic and use Reiter s example to form it into a default logic. Giving a node a default value would be equivalent to saying if it is consistent for this node to have that value then it does. Then we could use default theories to describe feature structures. The particular feature structure described would be the structure that supports the extension of the default theory. This is in effect what the theory of nonmonotonic sorts gives you. This paper describes how that theory derives from what is described above. INTRODUCTION There have been many suggestions for incorporating defaults into unification-based grammar formalisms Bouma 1990 Bouma 1992 Carpenter 1991 Kaplan 1987 Russell et al. 1992 Shieber 1986 Shieber 1987 . Each of these proposes a non-commutative non-associative default unification operation that combines one structure representing strict information with another representing default information. When presented with a set of structures the result depends on the order in which the structures are combined. This runs very much against the unification tradition in which any set has a unique most general satisfier if a satisfier exists at all . A method that is free of these ordering effects was presented in Young 1992 . The method of nonmonotonic sorts NSs allows default labels to be assigned at any time and used only in the absence of conflicting information. NSs replace the more traditional labels on feature structures to give nonmonotonically sorted feature structures NS-FSs . These structures can be combined by an associative and commutative unification operation. FSs are rederived from NSFSs by taking a solution an operation .

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