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<Paper uid="J87-1001">
  <Title>RESTRICTING LOGIC GRAMMARS WITH GOVERNMENT-BINDING THEORY</Title>
  <Section position="2" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 INTRODUCTION
</SectionTitle>
    <Paragraph position="0"> The best-known parser formalisms for logic programming systems have typically aimed to be expressive and efficient rather than restrictive. It is no surprise that in these systems a grammar writer can define linguistic structures that do not occur in any natural language. These unnatural structures might suffice for some particular processing of some particular fragment of a natural language, but there is a good chance that they will later need revision if the grammar needs to be extended to cover more of the natural language. On the other hand, if the grammar writer's options could be limited in the right way, there would be less to consider when a choice had to be made among various ways to extend the current grammar with the aim of choosing an extension that will not later need revision. Thus a restricted formalism can actually make it easier to build large, correct, and upward-compatible natural language grammars. A similar point obviously holds for automatic language learning systems. If a large class of languages must be considered, this can increase the difficulty of the (grammar induction) problem of correctly identifying an arbitrary language in the class. So there are certainly significant practical advantages to formalisms for natural language parsers that allow the needed linguistic structures to be defined gracefully while making it impossible to define structures that never occur.</Paragraph>
    <Paragraph position="1"> Recent work in linguistic theory provides some indications about how we can limit the expressive power of a grammar notation without ruling out any human languages. There appear to be severe constraints on the possible phrase structures and on the possible &amp;quot;movement&amp;quot; and &amp;quot;binding&amp;quot; relationships that can occur. The exact nature of these constraints is somewhat controversial. This paper will not delve into this controversy, but will just show how some of the constraints proposed recently by Chomsky and others - constraints to which all human languages are thought to conform can very easily be enforced in a parsing system that allows an elegant grammar notation. These grammars will be called restricted logic grammars (RLGs). Two well known logic grammar formalisms, definite clause grammars (DCGs) and extraposition grammars (XGs), will be briefly reviewed, and then RLGs will be introduced by showing how they differ from XGs. RLGs have a new type of rule (&amp;quot;switch rules&amp;quot;) that is of particular value in the definition of natural languages, and the automatic enforcement of some of Chomsky's constraints makes RLG movement rules simpler than XGs'. We follow the work of Marcus (1981), Berwick (1980), Wehrli (1984) and others in pursuing this strategy of restricting the grammar formalism by enforcing Chomsky's constraints, but we use a simple nondeterministic top-down back-tracking parsing method with lookahead, rather than Marcus's deterministic LR(k,t)-like parsing method. This approach to parsing, which has been developed in logic Copydght1987 by the Association for Computational Linguistics. Permission to copy without fee all or part of this material is granted provided that the copies are not made for direct commercial advantage and the CL reference and this copyright notice are included on the first page. To copy otherwise, or to republish, requires a fee and/or specific permission.  Computational Linguistics, Volume 13, Numbers 1-2, January-June 1987 1 Edward P. Stabler, Jr. Restricting Logic Grammars with Government-Binding Theory programming systems by Pereira and Warren (1980) and others, allows our rules to be very simple and intuitive.</Paragraph>
    <Paragraph position="2"> Since, on this approach, determinism is not demanded, we avoid Marcus's requirement that all ambiguity be resolved in the course of a parse.</Paragraph>
  </Section>
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