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<Paper uid="P88-1030">
  <Title>DEDUCTIVE PARSING WITH MULTIPLE LEVELS OF REPRESENTATION.*</Title>
  <Section position="3" start_page="0" end_page="242" type="intro">
    <SectionTitle>
INTRODUCTION
</SectionTitle>
    <Paragraph position="0"> This paper reports on several deductive parsers for a fragment of Chomsky's Government and Binding theory (Chomsky 1981, 1986; Van Riemsdijk and Williams 1984). These parsers were constructed to illustrate the 'Parsing as Deduction' approach, which views a parser as a specialized theorem-prover which uses knowledge of a language (i.e. its grammar) as a set of axioms from which information about the utterances of that language (e.g. their structural descriptions) can be deduced. This approach directly inspired by the seminal paper by Pereira and Warren (1983). Johnson (1988a) motivates the Parsing as Deduction approach in more detail than is possible here, and Johnson (1988b) extends the techniques presented in this paper to deal with a more complex fragment.</Paragraph>
    <Paragraph position="1">  Whitman Richards all made helpful suggestions regarding this work, although all responsibility for errors remains my own. The research reported here was supported by a grant by the Systems Development Foundation to the Center for the Study of Language and Information at Stanford University and a Postdoctoral Fellowship awarded by the Fairchild Foundation through the Brain and Cognitive Sciences Department at MIT.</Paragraph>
    <Paragraph position="2"> In this paper I describe a sequence of model deductive parsers, called PAD1 - PAD5, for a fragment of GB theory. These parsers are not designed for practical application, but simply to show that GB deductive parsers can actually be built. These parsers take PF representations as their input and produce LF representations as their output. They differ from most extant GB parsers in that they make explicit use of the four levels of representation that GB attributes to an utterance - namely D-structure, Sstructure, PF and LF - and the transformational relationship that holds between them. A &amp;quot;grammar&amp;quot; for these parsers consists entirely of a set of parameter values that parameterize the principles of GB theory - thus the parsers described here can be regarded as &amp;quot;principlebased' (Berwick 1987) - and the parsers' top-level internal structure transparently reflects (some of) the principles of that theory; X&amp;quot; and @ theory apply at D-structure, Case theory applies at S-structure, Move-or is stated as a relation between D- and S-structure, and LF-movement relates S-structure and LF. In particular, the constraints on S-structures that result from the interaction of Move-c~ with principles constraining D-structure (i.e. X' and @ theories) are used constructively throughout the parsingprocess.</Paragraph>
    <Paragraph position="3"> The PAD parsers are designed to directly mirror the deductive structure of GB theory.</Paragraph>
    <Paragraph position="4"> Intuitively, it seems that deductive parsers should be able to mirror theories with a rich internal deductive structure; these parsers show that to a first approximation this is in fact the case. For example, the PAD parsers have no direct specification of a 'rule' of Passive, rather they deduce the relevant properties of the Passive construction fi'om the interaction of O theory, Move-a, and Case theory.</Paragraph>
    <Paragraph position="5"> It must be stressed that the PAD parsers are only 'model' Parsers. The fragment of English they accept could only be called 'restricted'.</Paragraph>
    <Paragraph position="6"> They have no account of WH-movement, and Move-a is restricted to apply to lexical categories, for example, and they incorporate none of the principles of Bounding Theory.</Paragraph>
    <Paragraph position="7">  However, the techniques used to construct these parsers are general, and they should extend to a more substantial fragment.</Paragraph>
    <Paragraph position="8"> A SKETCH OF GB THEORY In the remainder of this section I sketch the aspects of GB theory relevant to the discussion below; for more detail the reader should consult one of the standard texts (e.g. Van Riemsdijk and Williams 1986). GB theory posits four distinct representations of an utterance, Dstructure, S-structure, PF and LF. To a first approximation, D-structure represents configurationally the thematic or predicate-argument structure of the utterance, S-structure represents the utterance's surface constituent structure, PF represents its phonetic form, and LF (&amp;quot;Logical Form&amp;quot;) is a configurational representation of the scopal relationships between the quantificational elements present in the utterance. The PF and LF representations constitute the interface between language and other cognitive systems external to the language module (Chomsky 1986, p. 68). For example, the PF representation &amp;quot;Everybody is loved&amp;quot; together with the D-structure, S-structure and LF representations shown in Figure 1 might constitute a well-formed quadruple for English.</Paragraph>
    <Paragraph position="9">  In order for such a quadruple to be well-formed it must satisfy all of the principles of grammar; e.g. the D-structure and S-structure must be related by Move(z, the D-structure must satisfy X'-theory and @-theory, etc. This is shown schematically in Figure 2, where the shaded rounded boxes indicate the four levels of representation, the boxes indicate relations that must hold simultaneously between pairs of structures, and the ellipses designate properties that must hold of a single structure. This diagram is based on the organization of GB theory sketched by Van Riemsdijk and Williams (1986, p. 310), and represents the organization of principles and structures incorporated in the parsers discussed below.</Paragraph>
    <Paragraph position="11"> Theory.</Paragraph>
    <Paragraph position="12"> The principles of grammar are parameterized; the set of structures they admit depends on the value of these parameters. These principles are hypothesised to be innate (and hence universally true of all human languages, thus they are often called &amp;quot;Universal Grammar'), so the extra knowledge that a human requires in order to know a language consists entirely of the values (or settings) of the parameters plus the lexicon for the language concerned. The syntax of the English fragment accepted by the parsers discussed below is completely specified by the following list of parameters. The first two parameters determine the X' component, the third parameter determines the Move-cz relation, and the fourth parameter identifies the direction of Case assignment.</Paragraph>
    <Paragraph position="13"> (1) headFirst.</Paragraph>
    <Paragraph position="14"> specFirst.</Paragraph>
    <Paragraph position="15"> movesInSyntax(np).</Paragraph>
    <Paragraph position="16"> rightwardCaseAssignment.</Paragraph>
    <Paragraph position="17"> I conclude this section with some brief remarks on the computational problems involved in constructing a GB parser. It seems that one can only construct a practical GB parser by simultaneously using constraints from all of the principles of grammar mentioned above (excepting LF-Movement), but this involves being able to &amp;quot;invert&amp;quot; Move-cz 'on the fly'. Because of the difficulty of doing this, most  implementations of GB parsers ignore Move-or entirely and reformulate X' and @ Theories so that they apply at S-structure instead of Dstructure, even though this weakens the explanatory power of the theory and complicates the resulting grammar, as Chomsky (1981) points out. The work reported here shows that it is possible to invert a simple formulation of Move-(x &amp;quot;on the fly', suggesting that it is possible to build parsers that take advantage of the D-structure/S-structure distinction offered by GB theory.</Paragraph>
  </Section>
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