Showing papers on "Indexed language published in 1998"

Contextual grammars as generative models of natural languages

Abstract: The paper discusses some classes of contextual grammars---mainly those with "maximal use of selectors"---giving some arguments that these grammars can be considered a good model for natural language syntax.A contextual grammar produces a language starting from a finite set of words and interatively adding contexts to the currently generated words, according to a selection procedure: each context has associated with it a selector, a set of words; the context is adjoined to any occurrence of such a selector in the word to be derived. In grammars with maximal use of selectors, a context is adjoined only to selectros for which no superword is a selector. Maximality can be defined either locally or globally (with respect to all selectors in the grammar). The obtained families of languages are incomparable with that of Chomsky context-free languages (and with other families of languages that contain linear languages and that are not "too large"; see Section 5) and have a series of properties supporting the assertion that these grammars are a possible adequate model for the syntax of natural languages. They are able to straightforwardly describe all the usual restrictions appearing in natural (and artificial) languages, which lead to the non-context-freeness of these languages: reduplication, crossed dependencies, and multiple agreements; however, there are center-embedded constructions that cannot be covered by these grammars.While these assertions concern only the weak generative capacity of contextual grammars, some ideas are also proposed for associating a structure to the generated words, in the form of a tree, or of a dependence relation (as considered in descrpitive linguistics and also similar to that in link grammars).

Linear indexed automata and tabulation of TAG parsing

Abstract: We present a new kind of recognizer for tree-adjoining languages, the linear indexed automata Such recognizers allow straightforward realization by means of logical programming languages We show that the computations by such automata, which are in general nondeterministic, can be tabulated by means of an extension of a technique originally devised for context-free languages The proposed application of this work is the design of efficient parsing algorithms for tree-adjoining grammars

An infinite hierarchy of mildly context-sensitive families of languages

Abstract: We prove the existence of an infinite hierarchy of mildly context-sensitive families of languages. The definition of this hierarchy is based on an extension of Marcus contextual grammars referred to as Marcus many dimensional simple contextual grammars. Some variants of this hierarchy are also considered. Interrelations between Marcus many dimensional simple contextual grammars and Chomsky grammars as well as interrelations with simple matrix grammars are studied. This research opens new directions to investigate various Marcus contextual grammars in the many dimensional case.

Learning a Subclass of Context-Free Languages

Abstract: In this paper, Apical growth Pure context-free grammars (AGPCFG) which are a variation of the pure grammars of Maurer et al are introduced. These grammars allow rewriting of active symbols at the ends of a string simultaneously. They also provide a variation of another kind of grammars called filamentous systems with apical growth, which are motivated by biological considerations. The family of languages generated by AGPCFGs is a subclass of context-free languages. An algorithm for learning this language subclass, in the framework of identification in the limit from positive examples, is provided.