TAILIEUCHUNG - Báo cáo khoa học: "Disjunctions and Inheritance in the Context Feature Structure System"

Substantial efforts have been made in order to cope with disjunctions in constraint based grammar formalisms (. [Kasper, 1987; Maxwell and Kaplan, 1991; DSrre and Eisele, 1990].). This paper describes the roles of disjunctions and inheritance in the use of feature structures and their formal semantics. With the notion of contexts we abstract from the graph structure of feature structures and properly define the search space of alternatives. The graph unification algorithm precomputes nogood combinations, and a specialized search procedure which we propose here uses them as a controlling factor in order to delay decisions as long as there. | Disjunctions and Inheritance in the Context Feature structure System Martin Bottcher GMD-IPSI Dolivostrafle 15 D 6100 Darmstadt Germany boettche@ Abstract Substantial efforts have been made in order to cope with disjunctions in constraint based grammar formalisms . Kasper 1987 Maxwell and Kaplan 1991 Dõrre and Eisele 1990 . . This paper describes the roles of disjunctions and inheritance in the use of feature structures and their formal semantics. With the notion of contexts we abstract from the graph structure of feature structures and properly define the search space of alternatives. The graph unification algorithm precomputes nogood combinations and a specialized search procedure which we propose here uses them as a controlling factor in order to delay decisions as long as there is no logical necessity for deciding. 1 Introduction The Context Feature Structure System CFS Bôttcher and Kônyves-Tóth 1992 is a unification based system which evaluates feature structures with distributed disjunctions and dynamically definable types for structure inheritance. CFS is currently used to develop and to test a dependency grammar for German in the text analysis project KONTEXT. In this paper disjunctions and inheritance will be investigated with regard to both their application dimension and their efficient computational treatment. The unification algorithm of CFS and the concept of virtual agreements for structure sharing has been introduced in Bõttcher and Kỏnyves-Tóth 1992 . The algorithm handles structure inheritance by structure sharing and constraint sharing which avoids copying of path structures and constraints completely. Disjunctions are evaluated concurrently without backtracking and without combinatoric multiplication of the path structure. For that purpose the path structure is separated from the structure of disjunctions by the introduction of contexts. Contexts are one of the key concepts for maintaining disjunctions in feature terms. They .

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