Lexical Acquisition in the Core Language Engine

LEXICAL ACQUISITION IN THE CORE LANGUAGE ENGINE David M. Carter SRI International Cambridge Research Centre 23 Millers Yard, Mill Lane Cambridge CB2 1RQ, U.K. Keywords: computational lexicography; lexical acquisition ABSTRACT The SRI Core Language Engine (CLE) is a general-purpose natural language front end for interactive systems. It trans- lates English expressions into representa- tions of their literal meanings. This paper , presents the lexical acquisition component of the CLE, which allows the creation of lexicon entries by users with knowledge of the application domain but not of linguis- tics or of the detailed workings of the sys- tem. It is argued that the need to cater for a wide range of types of back end leads naturally to an approach based on elic- iting grammaticality judgments from the user. This approach, which has been used to define a 1200-word core lexicon of En- • glish, is described and evaluated. 1 INTRODUCTION The SRI Core Language Engine (CLE; A1- shawi et al, 1988a,b) is a domain indepen- dent system for translating English sen- tences into formal representations of their literal meanings which are capable of sup- porting reasoning. It is designed to be used as a major component of interac- tive advisor systems such as interfaces to database management systems and diag- nostic expert systems. The main contri- bution of the CLE is intended to be sub- stantial coverage of English constructions in both syntax and semantics that is well motivated and hence extensible. The CLE makes use of three main types of lexicon entry. A syntactic entry for a word consists of one or more complex categories, each specified by a principal category symbol augmented by a set of con- straints on the values of syntactic fea- tures. Such categories also appear in the CLE's grammar, and match- ing and merging of the information encoded in them carried out by unifi- cation during parsing. Word sense entries for words are specified in the same way, but involve semantic as well as syntactic features. Semantic interpretation, which takes place in tandem with parsing, works by unification of feature values in word sense entries and semantic in- terpretation rules. Sortal (selectional) restrictions are defined for logical form predicates (i.e. word senses). After possi- ble semantic interpretations are con- structed, the CLE applies these re- strictions with reference to a user- definable hierarchy of sortal classes, to reject any interpretations in which the sort expected by some argument - 137 - of a predicate is inconsistent with that of the object filling that argu- ment. The CLE lexical acquisition tool VEX (for Vocabulary EXpander) allows the cre- ation of CLE lexicon entries by users with knowledge both of English and of the ap- plication domain, but not of linguistic the- ory or of the way lexical entries are rep- resented in the CLE. It asks the user for information on the grammaticality of ex- ample sentences, and for selectional re- strictions on arguments of predicates, and writes to disc a set of instructions …

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