Unification-based grammar formalisms use structures containing sets of features to describe linguistic objects. Although computational algo- rithms for unification of feature structures have been worked out in experimental research, these algorithms become quite complicated, and a more precise description of feature structures is desirable. We have developed a model in which descriptions of feature structures can be regarded as logical formulas, and interpreted by sets of directed graphs which satisfy them. These graphs are, in fact, transition graphs for a special type of deterministic finite automaton.

The design, implementation, and use of grammar formalisms for natural language have constituted a major branch of computational linguistics throughout its development. By viewing gramn, ar formalisms as just a special cause of computer languages, we can take advantage of the machinery of dcnoational semantics to provide a pre- cise specification of their meaning. Using Dana Scott's domain theory, we elucidate the nature of the feature systems used in augmented phrase-structure grammar formalisms, in particular those of recent versions of generalized phrase structure grammar, lexical functional grammar and PATRI1, and provide a dcnotational semantics for a simple gram- mar formalism. We find that the mathematical structures developed for this purpose contain an operation of feature generalization, not available in those grammar formalisms, that can be used to give a partial account of the effect of coordination on syntactic features.

In this paper, we argue that type inferencing incorrectly implements appropriateness specifications for typed feature structures, promote a combination of type resolution and unfilling as a correct and efficient alternative, and consider the expressive limits of this alternative approach. Throughout, we use feature cooccurence restrictions as illustration and linguistic motivation.

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. More and more often, computational linguists and language engineers need to manipulate large sets of feature structures which constitute real databases. Creating, updating, querying and in general, exploiting such linguistic databases is not an easy task, and it is desirable to have atone' disposal operators and languages for query and manipulation like (and if possible extending) those available for standard databases. The goal of this paper is to provide a database model for sets of feature structures, thus bridging the gap between the two fields, feature-structure theory and database theory, both of which are well-developed and well-formalized, but seem to have evolved in relatively separate communities. First, we show what feature structures have in common with, and how they differ from, the most classic data model, the relational model. We then formally define a database model for feature structures, and show how the standard operators of the relational data model can be general...

Although disjunction has been used in several unificationbased grammar formalisms, existing methods of unification have been unsatisfactory for descriptions containing large quantities of disjunction, because they require exponential time. This paper describes a method of unification by successive approximation, resulting in better average performance.