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<Paper uid="P95-1036">
  <Title>Some Novel Applications of Explanation-Based Learning to Parsing Lexicalized Tree-Adjoining Grammars&amp;quot;</Title>
  <Section position="3" start_page="268" end_page="268" type="intro">
    <SectionTitle>
2 Lexicalized Tree-Adjoining
Grammar
</SectionTitle>
    <Paragraph position="0"> Lexicalized Tree-Adjoining Grammar (LTAG) (Schabes et al., 1988; Schabes, 1990) consists of ELE-MENTARY TREES, with each elementary tree having a lexical item (anchor) on its frontier. An elementary tree serves as a complex description of the anchor and provides a domain of locality over which the anchor can specify syntactic and semantic (predicate-argument) constraints. Elementary trees are of two kinds - (a) INITIAL TREES and (b) AUX-ILIARY TREES.</Paragraph>
    <Paragraph position="1"> Nodes on the frontier of initial trees are marked as substitution sites by a '~'. Exactly one node on the frontier of an auxiliary tree, whose label matches the label of the root of the tree, is marked as a foot node by a '.'; the other nodes on the frontier of an auxiliary tree are marked as substitution sites. Elementary trees are combined by Substitution and Adjunction operations.</Paragraph>
    <Paragraph position="2"> Each node of an elementary tree is associated with the top and the bottom feature structures (FS). The bottom FS contains information relating to the sub-tree rooted at the node, and the top FS contains information relating to the supertree at that node. 1 The features may get their values from three different sources such as the morphology of anchor, the structure of the tree itself, or by unification during the derivation process. FS are manipulated by substitution and adjunction as shown in Figure 1.</Paragraph>
    <Paragraph position="3"> The initial trees (as) and auxiliary trees (/3s) for the sentence show me the flights from Boston to Philadelphia are shown in Figure 2. Due to the limited space, we have shown only the features on the al tree. The result of combining the elementary trees 1Nodes marked for substitution are associated with only the top FS.</Paragraph>
    <Paragraph position="4"> shown in Figure 2 is the derived tree, shown in Figure 2(a). The process of combining the elementary trees to yield a parse of the sentence is represented by the derivation tree, shown in Figure 2(b). The nodes of the derivation tree are the tree names that are anchored by the appropriate lexical items. The combining operation is indicated by the nature of the arcs-broken line for substitution and bold line for adjunction-while the address of the operation is indicated as part of the node label. The derivation tree can also be interpreted as a dependency tree 2 with unlabeled arcs between words of the sentence as shown in Figure 2(c).</Paragraph>
    <Paragraph position="5"> Elementary trees of LTAG are the domains for specifying dependencies. Recursive structures are specified via the auxiliary trees. The three aspects of LTAG - (a) lexicalization, (b)-extended domain of locality and (c) factoring of recursion, provide a natural means for generalization during the EBL proce88. null</Paragraph>
  </Section>
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