Discontinuous constituent in trees, rules, and parsing

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 phrase-structure 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 TENDUMdialogue system.