Syntactic and Semantic Knowledge in the DELPHI Unification Grammar

Syntactic and Semantic Knowledge in the DELPHI Unification Grammar R. Bobrow Robert Ingria David Stallard BBN Systems and Technologies Inc. 10 Moulton Street, Mailstop 6/4C Cambridge, MA 02138 Abstract This paper presents recent natural language work on HARC, the BBN Spoken Language System. The HARC system in- corporates the Byblos system [6] as its speech recognition component and the natural language system Delphi, which consists of a bottom-up parser paired with an integrated syn- tax/semantics unification grammar, a discourse module, and a database question-answering backend. The paper focuses on the syntactic and semantic analyses made in the grammar. Introduction This paper discusses the unification-based grammar compo- nent of the BBN Delphi system, the natural language compo- nent of the BBN Spoken Language System. This grammar, previously discussed in [4][5], is written in a variant of the Definite Clause Grammar formalism [17] and simultaneously assigns both syntactic structure and semantic interpretation to an utterance. The grammar comprises some 1300 rules (including those for terminal items), and has recently been evaluated on the ATIS air travel information domain with favorable results [2]. The paper has two main purposes. The first is to present this grammar's treatment of a number of syntactic and se- mantic phenonomena in the ATIS domain, in the belief that comparison of analyses are useful in and of themselves, par- ticularly in the context of a common evaluation. In particu- lar, we discuss the constraints imposed by subgrammars on conjunction, relaxation of subject verb agreement, optional argument subcategorization, and semantic interpretation of PPs and nominal compounds. The second purpose is to argue for the necessity of a de- vice which is frequently made use of Definite Clause Gram- mar work, but which has less often been utilized in the work on unification grammar that has appeared more recently ([19] [14]). This is the right-hand side "constraint relation" that does not derive any constituent of the utterance, but rather serves to constrain in various ways the feature assigments of those rule elements which do. Constraint relation elements are thus to be distinguished from other non-terminals which derive empty constituents, such as gaps or zero morphemes. While constraint relations have long been used in DCG- based workto treat, among other things, problems of quan- tifier scoping and the interaction of quanfifer scope and anaphora [16] [18] we argue that there are other, more low- level issues of lexical semantics and syntax for which con- straint relations are either conceptually useful or formally necessary, even in unification formalisms which allow full disjunction across features, such as [10] [12] [11] [9]. The remainder of the paper is divided into sections. In the next section, we first briefly review the formalism we use. In the two sections after that, we present syntactic analyses and semantic analyses made by the Delphi gram- mar, with emphasis placed on the use of constraint relation elements. Finally, in the last section we discuss the question of whether or not the constraint elements can be compiled out of grammar …

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