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<Paper uid="W06-1520">
  <Title>Handling Unlike Coordinated Phrases in TAG by Mixing Syntactic Category and Grammatical Function</Title>
  <Section position="3" start_page="0" end_page="137" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Generative grammars that we commonly hear about in computational linguistics are usually based on syntactic categories. This is also the case when the formalism used is the Tree Adjoining Grammars (TAGs). Large scale handcrafted grammars for many languages have been built based on this paradigm (Joshi, 2001; XTAG Research Group, 2001; Kroch and Joshi, 1985; Abeill*e and Candito, 2000; Candito, 1998; Becker, 1993; Frank, 2002; Joshi and Schabes, 1997; Abeill*e and Rambow, 2000) as well as grammars extracted from corpora (Chen and Vijay-Shanker, 2000; Chiang, 2000; Hwa, 1999; Xia et al., 2001; Xia, 2001). The latter is partly due to the fact that large scale annotated corpora such as the Penn Treebank (Marcus et al., 1994; Bies et al., 1995) give primacy to syntactic categories. After all this is the most strongly sedimented generative approach at least since (Chomsky, 1957).</Paragraph>
    <Paragraph position="1"> Computational approaches of grammar based on grammatical function such as that of Susumu Kuno (Kuno, 1987) have been given less importance. Although we can think of simply inserting functional labels in elementary trees or use them in a meta-level to generate the grammar, such as in (Candito, 1998; Kinyon, 2000; Cl*ement and Kinyon, 2003), such tags are generally not seen as an essential part of the derivational process.</Paragraph>
    <Paragraph position="2"> Nevertheless coordination is such an inherently functional phenomenon as we show next. Consider the sentences in (1) and (2). These are examples of regular coordination between phrases of the same category. They can easily be handled in the traditional grammar approaches of syntactic category.</Paragraph>
    <Paragraph position="3">  (1) She ew [PP on May 1st and on July 4th ].</Paragraph>
    <Paragraph position="4"> (2) They sell [ADJP electric and electronic ]  products.</Paragraph>
    <Paragraph position="5"> Now look at the cases in (3) and (4). They are different in the sense that the coordination is across categories. This poses a strong problem to the traditional grammar of syntactic categories. This has been noticed for TAGs in (Prolo, 2002). Recently this has also been tackled in the HPSG framework by (Sag, 2003) and (Abeill*e, 2004).</Paragraph>
    <Paragraph position="6"> The Penn Treebank calls this constituents UCP for Unlike Coordinated Phrases (Bies et al., 1995).</Paragraph>
    <Paragraph position="7"> The problem is that we would need rules of the kind below (using context-free rules for short see (Prolo, 2002) for TAGs). Basically all pairs of constituents can be coordinated but we can not assign to the resulting constituents either of the sub-constituent tags.</Paragraph>
    <Paragraph position="9"> (3) She ew [?? yesterday and on July 4th ].</Paragraph>
    <Paragraph position="10"> (4) They sell [?? electronic and computer ] de null vices.</Paragraph>
    <Paragraph position="11"> However, UCP coordination is not random. Two constituents can be coordinated only when they are ful lling the same grammatical function (with respect to a third head). In (3) they are playing the role of adverbial adjuncts of went. Either one can engage in that relation individually and hence they can be coordinated while playing that role. Likewise in (4) the adjective electronic and the noun computer are both ne as left NP modiers. Therefore they can be conjoined as such. As a nal example, consider the sentences in (5). Because the direct object of the verb know can be realized as either an NP or a sentential complement, they can be coordinated in that role as shown in (6).</Paragraph>
    <Paragraph position="12">  (5) I know the answer.</Paragraph>
    <Paragraph position="13"> I know that you don't know it.</Paragraph>
    <Paragraph position="14"> (6) I know [ the answer and that you don't know it ].</Paragraph>
    <Paragraph position="15">  Clearly the recursive process of conjoining the constituents is at the grammatic functional level. We show next how we can solve this problem elegantly by mixing grammatical function and syntactic category in the set of symbols for the tree nodes of a TAG.</Paragraph>
  </Section>
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