Multiple Default Inheritance in a Unification-Based Lexicon

Multiple Default Inheritance in a Unification-Based Graham Russell John Carroll* Susan Warwick-Armstrong ISSCO, 54 route des Acacias, 1227 Geneva, Switzerland elu@dlvsun.unige.ch Lexicon Abstract A formalism is presented for lexical specification in unification-based grammars which exploits defeasi- ble multiple inheritance to express regularity, sub- regularity, and exceptions in classifying the prop- erties of words. Such systems are in the general case intractable; the present proposal represents an attempt to reduce complexity while retaining suf- ficient expressive power for the task at hand. Illus- trative examples are given of morphological analy- ses from English and German. 1 Introduction The primary task of a computational lexicon is to associate character strings representing word forms with information able to constrain the distribution of those word forms within a sentence. 1 The or- ganization of a lexicon requires the ability, on the one hand, to make general statements about classes of words, and, on the other, to express excep- tions to such statements affecting individual words and subclasses of words. These considerations have provoked interest in applying to the lexicon AI knowledge representation techniques involving the notions of inheritance and default. 2 The sys- *current address: Cambridge University Computer Lab- oratory, New Museums Site, Pembroke Street, Cambridge CB2 3QG, UK. OWe are indebted to Af-.al Ballim, Mark Johnson, and anonymous referees for valuable comments on this paper. tin the general case, the relation between forms and in- formation is many-to-many (rather than one-to-many as of- ten assumed) and this observation has influenced the choice of facilities incorporated within the system. See 3.2 below for an example of how distinct forms share identical mor- phosyntactic specifications. 2See e.g. Daelemaus and Gazdar eds. (1990), and the references in Gazdar (1990). The work of Hudson (1984) extends this general approach to sentence syntax. tem described here is part of the ELU s unification grammar development environment intended for research in machine translation, comprising parser, generator, transfer mechanism and lexical compo- nents. The user language resembles that of PATR- II (Shieber, 1986), but provides a larger range of data types and more powerful means of stating re- lations between them. Among the requirements imposed by the context within which this system is used are (i) the ability to both analyse and gen- erate complex word forms, (ii) full integration with existing parts of the ELU environment, and (iii) the ability to accommodate a relatively large number of words. 2 Classes and Inheritance An ELU lexicon consists of a number of 'classes', each of which is a structured collection of con- straint equations and macro calls encoding infor- mation common to a set of words, together with links to other more general 'superc]asses'. For ex- ample, if an 'intransitive' class is used to express the common syntactic properties shared by all in- transitive verbs, then particular instances of in- transitive verbs can be made to inherit this infor- mation by specifying the 'intransitive' class as one of their superclasses - it then becomes unneces- saw to specify the relevant properties …

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