Analysis of Conjunctions in a Rule-Based Parser

ANALYSIS OF OONOUNCTIONS IN A ~JLE-~ PAKSER leonardo L~smo and Pietro Torasso Dipartimento di Informatica - Universita' di Torino Via Valperga Caluso 37 - 10125 Torino (ITALY) ABSTRACT The aim of the present paper is to show how a rule-based parser for the Italian language has been extended to analyze sentences involving conjunc- tions. The most noticeable fact is the ease with ~4nich the required modifications fit in the previ- ous parser structure. In particular, the rules written for analyzing simple sentences (without conjunctions) needed only small changes. On the contrary, more substantial changes were made to the e~oeption-handling rules (called "natural changes") that are used to restructure the tree in case of failure of a syntactic hypothesis. T0~ parser described in the present work constitutes the syn- tactic component of the FIDO system (a Flexible Interface for Database Operations), an interface allowing an end-user to access a relational data- base in natural language (Italian). INTRODUCTION It is not our intention to present here a comprehensive overview of the previous work on coordination, but just to describe a couple of recent studies On this topic and to specify the main differences between them and our approach. It must be noticed, however, that both systems that will be discussed use a logic grammar as their basic framework, so that we will try to make the comparison picking out the basic principles for the manipulation of conjunctions, and disregarding the more fundamental differences concerning the global system design. It is also worth pointing out that, although the present section is ac~nittedly incom- plete, most of the systems for the automatic analysis of I~ural language do not describe the met~hods adopted for the interpretation of sentences containing conjunctions in great detail. There- fore, it is reasonable to assume that in many of these systems the conjunctions are handled only by means of specific heuristic mechanisms. A noticeable e~ception is the ~ facility of the U/R~%R system (Woods, 1973): in this case, The research project described in this paper has partially been m/pported by the Ministero della Babblica Istruzione of Italy, MPI 40% Intelligenza Artificiale. the conjunctions are handled by m~ans of a para- syntactic mechanis~ that enables the parser to analyze the second conjunct assuming that it has a structure dependent on the hypothesized first con- junct. The main drawback of this approach is that the top-down bias of the ATNs does not allow the system to take advantage of the actual structure of the second conjunct to hypothesize its role. In other words, the analysis of the second conjunct acts as a confirnution mechanism for the hypothesis made on the sole basis of the position where the conjunction has been found. Consequently, all the v~rious possibilities (of increasing levels of com- plexity) must be analyzed until a match is found, which involves an apparent ~aste of computational resources. The solution proposed in the first of the systems we will be discussing here is quite simi- lar. It is based on Modifier Structure …

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