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<Paper uid="H89-2053">
  <Title>Joshi, Aravind K., 1985. How Much Context-Sensitivity is Necessary for Characterizing Structural Descriptions-- Tree Adjoining Grammars. In Dowty, D.; Karttunen, L.; and Zwicky, A. (editors), Natural Language Processing--</Title>
  <Section position="3" start_page="0" end_page="402" type="intro">
    <SectionTitle>
1 LEXICALIZED GRAMMARS
</SectionTitle>
    <Paragraph position="0"> Most current linguistic theories give lexical accounts of several phenomena that used to be considered purely syntactic. The information put in the lexicon is thereby increased in both amount and complexity: see, for example, lexical rules in LFG (Kaplan and Bresnan, 1983), GPSG (Gazdar, Klein, Pullum and Sag, 1985), HPSG (Pollard and Sag, 1987), Combinatory Categorial Grammars (Steedman 1985, 1988), Karttunen's version of Categorial Grammar (Karttunen 1986, 1988), some versions of GB theory (Chomsky 1981), and Lexicon-Grammars (Gross 1984).</Paragraph>
    <Paragraph position="1"> We say that a grammar is 'lexicalized' if it consists of.. 1 * a finite set of structures each associated with a lexical item; each lexical item will be called the anchor of the corresponding structure; the structures define the domain of locality over which constraints are specified; constraints are local with respect to their anchor; * an operation or operations for composing the structures.</Paragraph>
    <Paragraph position="2"> Notice that Categorial Grammars (as used for example by Ades and Steedman, 1982 and Steedman, 1985 and 1988) are 'lexicalized' according to our definition since each basic category has a lexical item associated with it.</Paragraph>
    <Paragraph position="3"> A general two-step parsing strategy for 'lexicalized' grammars follows naturally. In the first stage, the parser selects a set of elementary structures associated with the lexical items in the input sentence, and in the second stage the sentence is parsed with respect to this set. The strategy is independent of the nature of the elementary structures in the underlying grammar. In principle, any parsing algorithm can be used in the second stage.</Paragraph>
    <Paragraph position="4"> 1 By qexicalization' we mean that in each structure there is a lexical item that is realized. We do not mean simply adding feature structures (such as head) and unification equations to the rules of the formalism.</Paragraph>
    <Paragraph position="5">  The first step selects a relevant subset of the entire grammar, since only the structures associated with the words in the input string are selected for the parser. In the worst case, this filtering would select the entire grammar. The number of structures filtered during this pass depends on the nature of the input string and on characteristics of the grammar such as the number of structures, the number of lexical entries, the degree of lexical ambiguity, and the languages it defines.</Paragraph>
    <Paragraph position="6"> Since the structures selected during the first step encode the morphological value of their anchor (and therefore its position in the input string), the first step also enables the parser to use non-local information to guide its search. The encoding of the value of the anchor of each structure constrains the way the structures can be combined. It seems that this information is particularly useful for parsing algorithms that have some top-down behavior.</Paragraph>
    <Paragraph position="7"> This parsing strategy is general and any standard parsing technique can be used in the second step. Perhaps the advantages of the first step could be captured by some other technique. However this strategy is extremely simple and is consistent with the linguistic motivations for lexicalization.</Paragraph>
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