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<Paper uid="E95-1024">
  <Title>Off-line Optimization for Earley-style HPSG Processing</Title>
  <Section position="2" start_page="0" end_page="173" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Bidirectionality of grammar is a research topic in natural language processing that is enjoying increasing attention (Strzalkowski, 1993a). This is mainly due to the clear theoretical and practical advantages of bidirectional grammar use (see, among others, Appelt, 1987). We address this topic in describing a novel approach to HPSG (Pollard and Sag, 1994) based language processing that uses an off-line compiler to automatically prime a declarative grammar for generation or parsing, and hands the primed grammar to an advanced Earley processor. The developed techniques are direction independent in the sense that they can be used for both generation and parsing with HPSG grammars. In this paper, we focus on the application of the developed techniques in the context of the comparatively neglected area of HPSG generation.</Paragraph>
    <Paragraph position="1"> Shieber (1988) gave the first use of Earley's algorithm for generation, but this algorithm does not *The presented research was sponsored by 'l~eilpro jekt B4 &amp;quot;Constraints on Grammar for Efficient Generation&amp;quot; of the Sonderforschungsbereich 340 &amp;quot;Sprachtheoretische Grundlagen fiir die Computerllnguistik&amp;quot; of the Deutsche Forschungsgemeinschaft. The authors wish to thank Paul King, Detmar Meurers and Shuly Wintner for valuable comments and discussion. Of course, the authors are responsible for all remaining errors.</Paragraph>
    <Paragraph position="2"> use the prediction step to restrict feature instantiations on the predicted phrases, and thus lacks goaldirectedness. Though Gerdemann (1991) showed how to modify the restriction function to make top-down information available for the bottom-up completion step, Earley generation with top-down prediction still has a problem in that generating the sub-parts of a construction in the wrong order might lead to massive nondeterminacy or even nontermination.</Paragraph>
    <Paragraph position="3"> Gerdemann (1991) partly overcame this problem by incorpQrating a head-driven strategy into Earley's algorithm. However, evaluating the head of a construction prior to its dependent subparts still suffers from efficiency problems when the head of a construction is either missing, displaced or underspecified. Furthermore, Martinovid and Strzalkowski (1992) and others have observed that a simple head-first reordering of the grammar rules may still make insufficient restricting information available for generation unless the form of the grammar is restricted to unary or binary rules.</Paragraph>
    <Section position="1" start_page="0" end_page="173" type="sub_section">
      <SectionTitle>
Strzalkowski's Essential Arguments Approach
</SectionTitle>
      <Paragraph position="0"> (Ehh; 1993b) is a top-down approach to generation and parsing with logic grammars that uses off-line compilation to automatically invert parser-oriented logic grammars. The inversion process consists of both the automatic static reordering of nodes in the grammar, and the interchanging of arguments in rules with recursively defined heads. It is based on the notion of essential arguments, arguments which must be instantiated to ensure the efficient and terminating execution of a node. Minnen et al. (1995) observe that the EAA is computationally infeasible, because it demands the investigation of almost all possible permutations of a grammar. Moreover, the interchanging of arguments in recursive procedures as proposed by Strzalkowski fails to guarantee that input and output grammars are semantically equivalent. The Direct Inversion Approach (DI,~) of Minnen et al. (1995) overcomes these problems by making the reordering process more goal-directed and developing a reformulation technique that allows the successful treatment of rules which exhibit head-recursion. Both the EAA and the DIA were  presented as approaches to the inversion of parser-oriented grammars into grammars suitable for generation. However, both approaches can just as well take a declarative grammar specification as input to produce generator and/or parser-oriented grammars as in Dymetman et al. (1990). In this paper we adopt the latter theoretically more interesting perspective. null We developed a compiler for off-line optimization of phrase structure rule-based typed feature structure grammars which generalizes the techniques developed in the context of the DIA, and we advanced a typed extension of the Earley-style generator of Gerdemann (1991). Off-line compilation (section 3) is used to produce grammars for the Earley-style generator (section 2). We show that our use of off-line grammar optimization overcomes problems with empty or displaced heads. The developed techniques are extensively tested with a large HPSG grammar for partial vP topicallzation in German (iiinrichs et al., 1994). This uncovered some important constraints on the form of the phrase structure rules (phrase structure rules) in a grammar imposed by the compiler (section 4).</Paragraph>
    </Section>
  </Section>
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