A Uniform Approach to Underspecification and Parallelism

A Uniform Approach to Underspecification and Parallelism Joachim Niehren Programming Systems Lab Universitgt des Saarlandes Saarbrficken, Germany niehren©ps, uni- sb. de Manfred Pinkal Department of Computational Linguistics UniversitS~t des Saarlandes Saarbrficken, Germany pinkal@coli, uni- sb. de Peter Ruhrberg Department of Computational Linguistics Universit/it des Saarlandes Saarbrficken, Germany peru@coli, uni-sb, de Abstract We propose a unified framework in which to treat semantic underspecification and parallelism phenomena in discourse. The framework employs a constraint language that can express equality and subtree rela- tions between finite trees. In addition, our constraint language can express the equal- ity up-to relation over trees which cap- tures parallelism between them. The con- straints are solved by context unification. We demonstrate the use of our framework at the examples of quantifier scope, ellipsis, and their interaction. 1 1 Introduction Traditional model-theoretic semantics of natural languages (Montague, 1974) has assumed that se- mantic information, processed by composition and reasoning processes, is available in a completely specified form. During the last few years, the phe- nomenon of semantic underspecification, i.e. the incomplete availability of semantic information in processing, has received increasing attention. Sev- eral aspects of underspecification have been fo- cussed upon, motivated mainly by computational considerations: the ambiguity and openness of lex- ical meaning (Pustejovsky, 1995; Copestake and Briscoe, 1995), referential underspecification (Asher, 1993), structural semantic underspecification caused by syntactic ambiguities (Egg and Lebeth, 1995), and by the underdetermination of scope relations (Alshawi and Crouch, 1992; Reyte, 1993). In ad- dition, external factors such as insufficient coverage 1The research reported in this paper has been sup- ported by the SFB 378 at the UniversitS.t des Saarlandes and the Esprit Working Group CCL II (EP 22457). of the grammar, time-constraints for parsing, and most importantly the kind of incompleteness, uncer- tainty, and inconsistency, coming with spoken input are coming more into the focus of semantic process- ing (Bos et al., 1996; Pinkal, 1995). The aim of semantic underspecification is to pro- duce compact representations of the set of possible readings of a discourse. While the readings of a dis- course may be only partially known, the interpre- tations of its components are often strongly corre- lated. In this paper, we are concerned with a uni- form treatment of underspecification and of phenom- ena of discourse-semantic parallelism. Some typical parallelism phenomena are ellipsis, corrections, and variations. We illustrate them here by some exam- ples (focus-bearing phrases are underlined): (1) John speaks Chinese. Bill too. (2) John speaks Japanese. - No, he speaks Chinese. (3) ??? - Bill speaks Chinese, too. Parallelism guides the interpretation process for the above discourses. This is most obvious in the case of ellipsis interpretation (1), but is also evident for the resolution of the anaphor in the correction in (2), and in the variation case (3) where the context is unknown and has to be inferred. The challenge is to integrate a treatment of paral- lelism with underspecification, such as in cases of the interaction of scope and ellipsis. Problematic examples like (4) have been …

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