TAILIEUCHUNG - Báo cáo khoa học: "A Unification Method for Disjunctive Feature Descriptions"

Although disjunction has been used in several unificationbased grammar formalisms, existing methods of unification have been unsatisfactory for descriptions containing large quantities of disjunction, because they require exponential time. This paper describes a method of unification by successive approximation, resulting in better average performance. | A Unification Method for Disjunctive Feature Descriptions Robert T. Kasper USC Information Sciences Institute 4676 Admiralty Way Suite 1001 Marina del Rey CA 90292 and Electrical Engineering and Computer Science Department University of Michigan Abstract Although disjunction has been used in several unificationbased grammar formalisms existing methods of unification have been unsatisfactory for descriptions containing large quantities of disjunction because they require exponential time. This paper describes a method of unification by successive approximation resulting in better average performance. 1 Introduction Disjunction has been used in several unification-based grammar formalisms to represent alternative structures in descriptions of constituents. Disjunction is an essential component of grammatical descriptions in Kay s Functional Unification Grammar 6 and it has been proposed by Karttunen as a linguistically motivated extension to PATR-H 2 . In previous work two methods have been used to handle disjunctive descriptions in parsing and other computational applications. The first method requires expanding descriptions to disjunctive normal form DNF so that the entire description can be interpreted as a set of structures each of which contains no disjunction. This method is exemplified by Definite Clause Grammar 8 which eliminates disjunctive terms by expanding each rule containing disjunction into alternative rules. It is also the method used by Kay 7 in parsing FUG. This method works reasonably well for small grammars but it is clearly unsatisfactory for descriptions containing more than a small number of disjunctions because the DNF expansion requires an amount of space which is exponential in the number of disjunctions. The second method developed by Karttunen 2 uses constraints on disjuncts which must be checked whenever a disjunct is modified. Karttunen s method is only applicable to value disjunctions . those disjunctions used to specify the value of

• Báo cáo khoa học
• A Unification Method
• Disjunctive Feature Descriptions
• Robert T
• Kasper
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