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<Paper uid="C92-4174">
  <Title>DIRECT PARSING WITH METARUI~ES</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> 1tie metarules within the theory of Genendized Phrase Structure Grammar (GPSG) are a very interesting device to express generalizations on a basic set of immediate dominance (ID) rules. A short introduction to the theory of metarules is given in the following section. Metarules are used to generate an object gnanmar from a set of basic ID rules. One of the first discussions about the application time of metarules within the GPSG theory, which has strongly inlhlenced the succeeding research ill this field, has been published by \[Thompson 821. In his article Thompson argued fi~r applying the metarules all at once in a preprocessing step (compile-tilne application). However, our parer applies file metarules during tile parsing process (run-time aPldication or direct application). A discussion why we prefer file direct application is given in section 3. No matter when tile metarules are applied to tile basic set of ID rules, we have to care for tile termination of the recursive application of metarules. \[Thompson 82\] made a propo~l to guarantee termination by the definition of a Finite Closure (FC). This approach restricts the application of nlctarulcs such that one metarule can only be applied once in the derivation of an ID rule and prevents it from recnrsive al)l)licatiou. This restriction has been taken over by the authors of \[G~dar et Ill. 85\] and they gave a further restriction. They restricted the application of ntetarules to lexical ID rules, even though they knew that this restriction ulay prove to Ix; incompatible with the descriptive lx)wer needed for natural language gramnmrs (p. 59). But we think that there is nu need to restrict the application of mekarules only to lexical ID rules, even if there are proposals to elimi~mte file u~ of metarules and to use lexical rules like in LFG and HPSG (see \[Jacobson 87\]). But to do so with GPSG would involve crucial changes u~ tile theory, and therefore we preserved file metarule component ill our machine translation (MT) system aml tried to find an adequate criterion h)l tile termination of melarules. In our approach die granlnlar writer is lice to decide whether a metarule is to be applied to lexical ID rules or to another type of rule.</Paragraph>
    <Paragraph position="1"> Tile Finite Closure (FC) is too restricive, \[w.,cause in some cases (see tile examples of IUszkoreit 87\] and \[G~dar et ill. 851 in tile sections 3 and 4, respectively) re.cursive application is needed. In section 4 we present all alternative constraint on die basis of results in the lield of term-rewriting. &amp;quot;l-lais constraint is less restrictive than the FC. It 'allows for the definition of recursive metarules which may be applied freely and guarantees the termination. In section 5 the metarules of the German GPSG grammar used in our MT system are outlined and, with the help of the constraint in section 4, we show that their application is terminating.</Paragraph>
    <Paragraph position="2"> hi section 6 we give an outline of how to modify the parsing process in a way that nretarules can be applied directly rather than at compile-time. In the last section we draw sonre conclusions for future work in this field.</Paragraph>
  </Section>
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