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Báo cáo khoa học: "DISCONTINUOUS CONSTITUENTS IN TREES, RULES, AND PARSING"

This paper discusses the consequences of allowing discontinuous constituents in syntactic representions and phrase-structure rules, and the resulting complications for a standard parser of phrase-structure grammar. It is argued, first, that discontinuous constituents seem inevitable in a phrase-structure grammar which is acceptable from a semantic point of view. It is shown that tree-like constituent structures with discontinuities can be given a precise definition which makes them just as acceptable for syntactic representation as ordinary trees. However, the formulation of phrase-structure rules that generate such structures entails quite intricate problems. . | DISCONTINUOUS CONSTITUENTS IN TREES RULES AND PARSING Harry Bunt Jan Thesingh and Ko van der Sloot Computational Linguistics Unit Tilburg University SLE Postbus 90153 5000 LE TILBURG The Netherlands ABSTRACT This paper discusses the consequences of allowing discontinuous constituents in syntactic representions and phrase-structure rules and the resulting complications for a standard parser of phrase-structure grammar. It is argued first that discontinuous constituents seem inevitable in a phrase-structure grammar which is acceptable from a semantic point of view. It is shown that tree-like constituent structures with discontinuities can be given a precise definition which makes them just as acceptable for syntactic representation as ordinary trees. However the formulation of phrase-structure rules that generate such structures entails quite intricate problems. The notions.of linear precedence and adjacency are reexamined and the concept of n-place adjacency sequence is introduced. Finally the resulting form of p h r a s e - s t r u cture grammar called Discontinuous Phrase-Structure Grammar is shown to be parsable by an algorithm for context-free parsing with relatively minor adaptations. The paper describes the adaptations in the chart parser which was implemented as part of the TENDUM dialogue system. 1. Phrase-structure grammar and discontinuity Context-free phrase-structure grammars PSGs have always been popular in computational linguistics and in the theory of programming languages because of their technical and conceptual simplicity and their well-established efficient parsability Shell 1976 Tomita 1985 . In theoretical linguistics it was generally believed until recently that natural language competence cannot be characterized adequately by a context-free grammar especially in view of agreement phenomena and discontinuities see e.g. Postal 1964 . However in the early eighties Gazdar and others revived an idea due to Harman 1 963 of formulating .