8 - A Practical Approach to Multiple Default Inheritance for Unification-Based Lexicons

Summary

Introduction

Natural language lexicons form an obvious application for techniques involving default inheritance developed for knowledge representation in artificial intelligence (AI). Many of the schemes that have been proposed are highly complex – simple tree-form taxonomies are thought to be inadequate, and a variety of additional mechanisms are employed. As Touretzky et al. (1987) show, the intuitions underlying the behaviour of such systems may be unstable, and in the general case they are intractable (Selman and Levesque, 1989).

It is an open question whether the lexicon requires this level of sophistication – by sacrificing some of the power of a general inheritance system one may arrive at a simpler, more restricted, version, which is nevertheless sufficiently expressive for the domain. The particular context within which the lexicon described here has been devised seems to permit further reductions in complexity. It has been implemented as part of the ELU unification grammar development environment for research in machine translation, comprising parser, generator, lexicon, and transfer mechanism.

Overview of Formalism

An ELU lexicon consists of a number of ‘classes’, each of which is a structured collection of constraint equations and/or macro calls encoding information common to a set of words, together with links to other more general ‘superclasses’. Lexical entries are themselves classes, and any information they contain is standardly specific to an individual word; lexical and non-lexical classes differ in that analysis and generation take only the former as entry points to the lexicon.