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<Paper uid="W98-0504">
  <Title>r g ! How to define a context-free backbone for DGs: Implementing a DG in the LFG formalism</Title>
  <Section position="2" start_page="0" end_page="29" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Recently, the concept of valency has gained considerable attention. Not only do all linguistic theories refer to some reformulation of the traditional notion of valency (in the form of 0grid, subcategorization list, argument list, or extended domain of locality); there is a growing number of parsers based on binary relations between words (Eisner, 1997; Maruyama, 1990).</Paragraph>
    <Paragraph position="1"> Even theories based on phrase structure may have processing models based on relations between lexical items (Rambow &amp; Joshi, 1994).</Paragraph>
    <Paragraph position="2"> Against the background of this interest in the valency concept, and the fact that word order is one of the main difference between phrase-structure based approaches (henceforth PSG) and dependency grammar (DG), this paper will propose a word order description for DG and describe its implementation. First, we will motivate the separation of surface order and dependency relations within DG, and make a specific architectural proposal for their linking. Second, we will briefly sketch Lexical-Functional Grammar (LFG), and then show in detail how one might use the formal constructs provided by LFG to encode the proposed DG architecture.</Paragraph>
    <Paragraph position="3"> Our position will be that dependency relations are motivated semantically (Tesni~re, 1959), and need not be projective. We argue for so-called word order domains, consisting of partially ordered sets of words and associated with nodes in the dependency tree. These order domains constitute a tree defined by set inclusion, and surface word order is determined by traversing this tree. A syntactic analysis therefore consists of two linked, but dissimilar trees. The paper thus sheds light on two questions.</Paragraph>
    <Paragraph position="4"> A very early result on the weak generative equivalence of context-free grammars and DGs suggested that DGs are incapable of describing surface word order (Gaifman, 1965). J This result has been criticised to apply only to impoverished DGs which do not properly represent formally the expressivity of contemporary DG variants (Neuhaus &amp; BrSker, 1997), and our use of a context-free backbone with further constraints imposed by dependency relations further supports the view that DG is not a notational ~riant of context-free grammar. The second question addressed is that of efficient processing of discontinuous DGs. By converting a native DG grammar into LFG rules, we are able to profit from the state of the art in context-free parsing technology. A context-free base (or skeleton) has often been cited as a prerequisite for practical applicability of a natural language grammar (Erbach &amp; Uszkoreit, 1990), and we here show that a DG can meet this criterion with ease.</Paragraph>
    <Paragraph position="5"> Sec. 2 will briefly review approaches to word order in DG, and Sec. 3 introduces word order domains as our proposal. LFG is briefly introduced in Sec. 4, and the encoding of DG within the LFG framework is the topic of Sec. 5.</Paragraph>
    <Paragraph position="7"/>
  </Section>
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