Representing a System of Lexical Types Using Default Unification

Proceedings of EACL '99 Representing a System of Lexical Types Using Default Unification Aline Villavicencio Computer Laboratory University of Cambridge New Museums Site Pembroke Street Cambridge CB2 3QG ENGLAND Aline.Villavicencio@cl.cam.ac.uk Abstract Default inheritance is a useful tool for encoding linguistic generalisations that have exceptions. In this paper we show how the use of an order independent typed default unification operation can provide non-redundant highly structured and concise representation to specify a network of lexical types, that encodes linguistic information about verbal sub- categorisation. The system of lexical types is based on the one proposed by Pollard and Sag (1987), but uses the more expressive typed default feature structures, is more succinct, and able to express linguistic sub-regularities more elegantly. 1 Introduction Several authors have highlighted the importance of using defaults in the representation of linguistic knowledge, in order to get linguistically adequate descriptions for some natural language phenom- ena ((Gazdar, 1987), (Bouma, 1992), (Daelemans et al, 1992), (Briscoe, 1993)). Defaults have been used in the definition of inflectional morphology, specification of lexical semantics, analysis of gap- ping constructions and ellipsis among others. In this paper we use defaults to structure the lexicon, concentrating on the description of verbal subcat- egorisation information. The issue of how to organise lexical informa- tion is especially important when a lexicalised for- malism like Categorial Grammar (CG) or Head- Driven Phrase Structure Grammar (HPSG) is em- ployed, since the burden of linguistic description is concentrated in the lexicon and if lexical en- tries are organised as unrelated lists, there is a significant loss of generalisation and an increase in redundancy. Alternatively, it is possible to use inheritance networks, which provide representa- tions that are able to capture linguistic regulari- ties about classes of items that behave similarly. This idea is employed in Pollard and Sag's (1987) sketch of an HPSG lexicon as a monotonic mul- tiple orthogonal inheritance type hierarchy. How- ever, this work fail to make use of defaults, which would significantly reduce redundancy in lexical specifications and would enable them to elegantly express sub-regularities (Krieger and Nerbonne, 1993). In this paper we demonstrate that using default unification, namely the order-independent and persistent version of default unification de- scribed in (Lascarides et al, 1996b) and (Las- carides and Copestake, 1999), to implement a de- fault inheritance network results in a fully declar- ative specification of a lexical fragment based on Pollard and Sag's (1987), but that is both more succinct and able to express elegantly linguistic sub-regularities, such as the marked status of sub- ject control of transitive subject-control verbs. In section 2, a brief description of the use of de- faults and YADU is given. In section 3, we present the results of representing the proposed lexical fragment in terms of default multiple inheritance networks. Finally, we discuss the results achieved and future work. 2 Default Inheritance and YADU In this work, a default multiple orthogonal inher- itance network is used to represent lexical infor- mation. Thus, with different subnetworks used to encode …

Content truncated to 500 words