The limits of unification

Xem 1-7 trên 7 kết quả The limits of unification
  • Current complex-feature based grammars use a single procedure--unification--for a multitude of purposes, among them, enforcing formal agreement between purely syntactic features. This paper presents evidence from several natural languages that unification--variable-matching combined with variable substitution--is the wrong mechanism for effecting agreement. The view of grammar developed here is one in which unification is used for semantic interpretation, while purely formal agreement involves only a check for non-distinctness---i.e, variable-matching without variable substitution. ...

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  • The notion of a Horn extended feature structure (HoXF) is introduced, which is a feature structure constrained so that its only allowable extensions are those satisfying some set of llorn clauses in featureterm logic, l l o X F ' s greatly generalize ordinary feature structures in admitting explicit representation of negative and implicational constraints. In contradistinction to the general case in which arbitrary logical constraints are allowed (for which the best known algorithms are exponential), there is a highly tractable algorithm for the unification of HoXF's. ...

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  • 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 while preserving the desirable properties of FUGs.

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  • This paper shows how higher levels of generalization can be introduced into unification grammars by exploiting methods for typing grammatical objects. We discuss the strategy of using global declarations to limit possible linguistic structures, and sketch a few unusual aspects of our typechecking algorithm. We also describe the sort system we use in our semantic representation language and illustrate the expressive power gained by being able to state global constraints over these sorts.

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  • In order to realize their full potential, multimodal systems need to support not just input from multiple modes, but also synchronized integration of modes. Johnston et al (1997) model this integration using a unification operation over typed feature structures. This is an effective solution for a broad class of systems, but limits multimodal utterances to combinations of a single spoken phrase with a single gesture. We show how the unification-based approach can be scaled up to provide a full multimodal grammar formalism.

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  • A common feature of recent unificationbased g r a m m a r formalisms is that they give the user the ability to define his own structures. However, this possibility is mostly limited and does not include nonmonotonic operations. In this paper we show how nonmonotonic operations can also be user-defined by applying default logic (Reiter, 1980) and generalizing previous results on nonmonotonic sorts (Young and Rounds, 1993).

    pdf7p bunmoc_1 20-04-2013 34 2   Download

  • One of the claimed benefits of Tree Adjoining G r a m m a r s is that they have an extended domain of locality (EDOL). We consider how this can be exploited to limit the need for feature structure unification during parsing. We compare two wide-coverage lexicalized g r a m m a r s of English, LEXSYS and XTAG, finding that the two grammars exploit EDOL in different ways.

    pdf8p bunthai_1 06-05-2013 39 2   Download



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