The unification problem and several variants are presented. Various algorithms and data structures are discussed. Research on unification arising in several areas of computer science is surveyed, these areas include theorem proving, logic programming, and natural language processing. Sections of the paper include examples that highlight particular uses

A distinctive propertyofmanycurrent grammatical formalisms is their use of feature equality constraints to express a wide variety of grammatical dependencies. Lexical-Functional Grammar[6], Head-Driven Phrase-Structure Grammar[14], PATR[8], FUG[12, 13], and the various forms of categorial uni cation grammar[9,15,16] all require an analysis of a sentence

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tem of rewrite rules or "productions" like (2), which have their origin in early work in recursion theory by Post, among others. (1) Dexter likes Warren. (2) S ! NP VP VP ! TV NP TV ! flikes;sees; : : :g Categorial Grammar (CG), together with its close cousin Dependency Grammar (which also originated in the 1950s, in work by Tesniere) stems from an alternative approach to context-free grammar pioneered by Bar-Hillel 1953 and Lambek 1958, with earlier antecedents in Ajdukiewicz 1935 and still earlier work by Husserl and Russell in category theory and the theory of types. Categorial Grammars capture the same information by associating a functional type or category with all grammatical entities. For example, all transitive verbs are associated via the lexicon with a category that can be written as follows: (3) likes := (SnNP)=NP The no

this paper, we study the problem of lexicalizing context-free grammars and show that it enables faster processing. In previous attempts to take advantage of lexicalization, a variety of lexicalization procedures have been developed that convert context-free grammars (CFGs) into equivalent lexicalized grammars. However, these procedures typically suffer from one or more of the following problems

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No assumption is more fundamental in the theory (and practice) of syntax than

Functional Unification Grammars (FUGs) are popular for natural language applications because the formalism uses very few primitives and is uniform and expressive. In our work on text generation, we have found that it also has annoying limitations: it is not suited for the expression of simple, yet very common, taxonomic relations and it does not allow the specification of completeness conditions. We have implemented an extension of traditional functional unification. This extension addresses these limitations whiIe preserving the desirable properties of FUGs. It is based on the notions of typed features and typed constituents. We show the advantages of this extension in the context of a grammar used for text generation.

this paper, we will study the possibilities for applying lexical rules to the analysis of English syntax, and in particular the structure of the verb phrase. We will develop a lexicon whose empirical coverage extends to the full range of verb subcategories, complex adverbial phrases, auxiliaries, the passive construction, yes/no questions and the particularly troublesome case of predicatives. The effect of a lexical rule, in our system, will be to produce new lexical entries from old lexical entries. The similarity between our system and the metarule system of generalized phrase-structure grammar (GPSG, as presented in Gazdar, et al. 1985) is not coincidental. Our lexical rules serve much the same purpose as metarules in GPSG, which were restricted to lexical phrase structure rules. The similarity is in a large part due to the fact that with the universal phrase-structure schemes being fixed, the role of a lexical category assignment in effect determines phrase-structure in much the same way as a lexical category entry and lexical phrase-structure rule determines lexical phrase-structure in GPSG. Our lexical rules will also bear a relationship to the lexical rules found in lexical-functional grammar (LFG, see Bresnan 1982), as LFG rules are driven by the grammatical role assigned to arguments. Many of our analyses were first applied to either LFG or GPSG, as these were the first serious linguistic theories based on a notion of unification. In the process of explaining the basic principles behind categorial grammar and developing our lexical rule system, we will establish a categorial grammar lexicon with coverage of English syntactic constructions comparable to that achieved within published accounts of the GPSG or LFG frameworks. Language, at its most abstract level, i...