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<Paper uid="C02-1153">
  <Title>Generating the XTAG English grammar using metarules</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
2 Metarules
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    <Paragraph position="0"> We present in this section an introductory example of metarules.4 Consider the two trees in Figure 3 3This work started years ago, already mentioned in (Doran et al., 2000, p. 388). There has been some confusion on the issue, perhaps driven by a somewhat ambiguous statement in (Becker, 2000, p. 331): &amp;quot;In this paper, we present the various patterns which are used in the implementation of metarules which we added to the XTAG system (Doran et al. 2000)&amp;quot;. The work of Becker conceived and developed the idea of metarules for TAGs (Becker, 1993; Becker, 1994). He also created the original implementation of the metarule interpreter as part of the XTAG software, from 1993 to 1995, thereafter improved to reach a first stable form as documented in (XTAG Research Group, 1998). However, with respect to grammar development, he only created the necessary example patterns to support the concepts of metarules, while the work described here is the first to actually evaluate metarules in-the-large as part of the XTAG project (a preliminary version of this paper was in the TAG+6 workshop).</Paragraph>
    <Paragraph position="1"> 4For a more comprehensive introduction of its linguistic motivations and the basic patterns it allows, see (Becker, 2000). anchored by verbs that take as arguments an NP and a PP (e.g., put).</Paragraph>
    <Paragraph position="2"> The one to the left corresponds to its declarative structure; the other to the wh-subject extracted form. Despite their complexity, they share most of their structure: the only differences being the whsite in the right tree (higher NP) and the trace at sub-ject position. That observation would not be very useful if the differential description we have made was idiosyncratic to this pair, which is not the case.</Paragraph>
    <Paragraph position="3"> Clearly, many other pairs all over the grammar will share the same differential description.</Paragraph>
    <Paragraph position="4">  Figure 4 shows a metarule for wh-subject extraction that captures the similarities mentioned above. It describes how to automatically generate the tree in Figure 3.b, given as input the tree in Figure 3.a.</Paragraph>
    <Paragraph position="5"> Here is how it works. First the input tree has to match the left-hand side of the metarule, lhs in Figure 4, starting from their roots. In the example, the lhs tree requires the candidate tree to have its root labeled a6a8a7 . Then, its leftmost child has to be an NP, as indicated by the node a9a11a10a13a12a15a14a17a16 in lhs: a9a11a10 indicates it is the variable a18a19a10 ; a12a20a14a21a16 indicates we need an NP, regardless of the subscript. Next, the lhs tree requires the rest of the tree to match variable a9a13a22 . That is trivial, because such variables with just an identification number are &amp;quot;wild cards&amp;quot; that match any range of subtrees. The matches of each variable in lhs, for the application to the input tree in Figure 3.a, are shown in Figure 5.</Paragraph>
    <Paragraph position="6"> Had the matching process failed no new tree would have been generated. Since in the example above the matching succeeded, the processor move  to the final step, which is to generate the new tree.</Paragraph>
    <Paragraph position="7"> We look at the right-hand side of the metarule rhs and just replace the instances of the variables there with their matched values, obtaining the tree in Figure 3.b. The same process can be applied for the many other pairs related by the same metarule.</Paragraph>
    <Paragraph position="8"> In a feature-based grammar as the one we are focusing on, to create tree structures without the proper feature equations is of little use. On the other hand, experience has shown that feature equations are much harder to maintain correct and consistent in the grammar than the tree structures. The XTAG metarules use features in two ways: as matching requirements, and for transformation purposes.</Paragraph>
    <Paragraph position="9"> 3 An ordered set of metarules The set of verbal trees can be seen as a subset of the Cartesian product of three dimensions: sub-categorization (e.g., transitive, intransitive), redistribution (e.g., passive), and realization (e.g., wh-subject movement) - discounted, of course, combinations blocked by linguistic constraints (e.g., there can not be object movement in intransitives). The verb trees in the XTAG English grammar are organized in families that roughly reflect a subcategorization frame. Hence, each family contains trees</Paragraph>
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