Controlling Lexical Substitution in Computer Text Generation

Controlling Lexical Substitution in Computer Text Generation 1 Robert Granville MIT Laboratory for Computer Science 545 Technology Square Cambridge, Massachusetts 02139 Abstract Th=s report describes Paul, a computer text generation system desig~ed LO create cohesive text through the use o| lexlcal substitutions. Specihcally, Ihas system is designed Io determmistically choose between provluminahzat0on, superordinate suhstntut0on, and dehmte noun phrase reiterabon. The system identities a strength el antecedence recovery for each of the lex~cal subshtutions, and matches them against the strength el potenfml antecedence of each element m the text to select the proper substitutions for these elements. 1. Introduction This report descrnbes Paul. a computer text generation system designed to cre~:te collesive text through tile use of lexical substitutuons. Spec;hcalty. thts system ~s designed tn deterministically choose between pronominal:zabon sup(:rordinate substitution, and delinite noun phrase reitcrat}on. Fl~e system identifies a strength at antecedence recovery for each of the lexical substitutions, anti matches them against the strength of potenU,# entececJence of each element =n the text to select the proper sub3litubons for these elements. P~ul is a natural language generation program initially developed at IBM's Thomas J. Watson Research Center as part of the ongoing Epistle project I5.6}, "[he emphasis of the the work reported here is in the research oJ discourse phenomena, the study of cohesion and its effects on mLJlhsententiat texts [3, 9]. Paul accepts as input LISP knowledge structures consisbng of case frame l1] formalisms representing each sentence to be gernerated. These knowledge structures are translated into Enghsh, with the appropriate lexical substitutions being made at this time. No attempt vs made by the system to create these knowledge structures. 2. Cohesion The purpose of communication is for one person (the speaker or writer) to express her thoughts and ideas so that another (the listener or reader) can understand them. ]here aJe many restrictions placed on the realization of these thoughts inio language so that the listener may understand. One ot the most important requiroments fo~ an utterance is that it seem to be unified, that it form a text. The theory of text and what distinguishes it from isolated sentences that is used in Paul is that of Halliday and Hasan [3]. One of the items that enhances the unity of text is cohesion. Cohesion refers to the linguistic phenomena that establish relationships between sentences, thc~reby tying them together. There are two major goals that are accomplished tl~rougi~ cohesiu, that enhance a passage's qualily of text. The fiust is the obwous oesure to avoid unnecessary repetibon. The other goal is to dislinguL",h new information from old. ,so that the listener can tully undemtand what fs being said. [1} The room has a large window, The room has a window facing east. {1} appears to he describing two windows, because there is no device indicating that the window of the second sentence is the same as the window of tile first sentence. If in tact the speaker me:mr to describe the stone w;ndow, silo must somehow inform the listener …

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