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<Paper uid="C90-2050">
  <Title>A Head-Driven Approach to Incremental and Parallel Generation of Syntactic Structures</Title>
  <Section position="2" start_page="0" end_page="0" type="abstr">
    <SectionTitle>
1. Introduction
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    <Paragraph position="0"> Incremental generation (i.e. immediate verbalization of the parts of a stepwise computed conceptual structure - often called &amp;quot;message&amp;quot;) is an important and efficient property of human language use (\[DeSmedt&amp;Kempen~7\], \[Levelt89\]). There are particular situations of conmmnication (e.g. simultaneous descriptions of ongoing events) where incremental generation is necessary in order to ensure that new information can be verbalized in due time. As \[DeSmedt&amp; Kempen87\] mentioned, incremental generation can be viewed as a parallel process: While the linguistic module (or &amp;quot;how-to-say&amp;quot; component) of an incremental generator is processing partial conceptual structures which are previously computed from the conceptual module (or &amp;quot;what-to-say&amp;quot; component) the latter can run 1Major parts of the work presented in this paper were developed during the master's thesis of the authors. This work was supported by the German Science Foundation (DFG) in its Special Collaborative Program on AI and Knowledge-Based Systems (SFB 314), project XTRA.</Paragraph>
    <Paragraph position="1"> Thanks to Gregor Erbach, Norbert Reithinger and Harald &amp;quot;Frost for their helpful comments on earlier versions of the paper.</Paragraph>
    <Paragraph position="2"> simultaneously and add more conceptual elements. In \[Finkler&amp;Neumann89\] we show that it is possible to refine this parallelism: Every conceptual or linguistic segment can be viewed as an active unit which tries to verbalize itself as fast and as independently as possible.</Paragraph>
    <Paragraph position="3"> If the translation of a segment is not possible because of unspecified but required information, it is necessary to request missing information in order not to jeopardize the fast mapping. As a consequence, the linguistic module provides feedback for the selection of what to say next by the conceptual module (cf.</Paragraph>
    <Paragraph position="4"> \[Hovy87\], \[Reithinger88\]).</Paragraph>
    <Paragraph position="5"> Incremental generation imposes special requirements upon syntactic description and processing (cf. \[Kempen87\]). Until now only a few approaches to syntax have been developed which consider explicitly the requirements of incremental construction of sentences, namely that of \[Kempen87\] and \[DeSmedt&amp; Kempen88\]. But those do not provide for a bidirectional flow of control between conceptual and linguistic module.</Paragraph>
    <Paragraph position="6"> In this paper we describe the principles of representation and processing of syntactic knowledge in the natural language generation system POPEL-HOW, which for the first time combines explicitly incremental multi-level generation with parallelism and feedback. The syntactic knowledge is declaratively represented  by a unification-based head-driven grammar that is linguistically based on dependency grammar.</Paragraph>
    <Paragraph position="7"> 2. Requirements upon the Syntactic  Level The following aspects concerning the representation of syntactic knowledge and the basic mechanism have to be regarded in an incremental multi-level and parallel model: 1. The grammar should be lexically based. The lexicon is assumed to be the &amp;quot;essential mediator&amp;quot; between conceptualization and grammatical encoding (cf. \[Levelt89\]). During incremental generation it is not plausible to assume that the syntactic structure is built up &amp;quot;from phrases to lexical elements&amp;quot; starting with a root node S.</Paragraph>
    <Paragraph position="8"> 2. The grammar should support vertical rather than horizontal orientation \[Kempen87\]. The rules should emphasize the growing of syntactic structures by individual branches (ideally, only by one branch, but see 4.1). One should avoid that, because of adding a new segment, unnecessary constraints are simultaneously added for sister segments.</Paragraph>
    <Paragraph position="9"> 3. Syntactic segments should be expanded in the following three ways \[Kempen87\]: upward (a new segment B becomes the root node of a previous segment A), downward (B becomes a daughter node of A) and insertion (B becomes the daughter node of A which already has a daughter and this daughter becomes the daughter of B) 2.</Paragraph>
    <Paragraph position="10"> 4. During incremental generation one should not assume that the chronological order in which syntactic segments are attached corresponds to the linear order of the resulting utterance. Hence one should separate knowk:dge concerning immediate dominance and linear precedence \[Kempen87\]. Especially during the pm'allel processing of languages with a relatively free word order (cog., German), one should avoid building up unnecessary syntactic paraphrases resulting tiom ordering voxiations.</Paragraph>
    <Paragraph position="11"> 5. When the whole syntactic stn.,cture of an utterance is built up in a parallel fashion, it should be possible to decide for every partial structure whether it is locally complete. In a head-driven grammar this is possible if segments are based on head elements. Thus only the information that is necessary for the inflection and linearization of the head is considered (see 4.2).</Paragraph>
    <Paragraph position="12"> 6. During spontaneous speech it may happen that already existing structures need to be modified because of new information that can only be considered by replacing old one (which means &amp;quot;reformulation&amp;quot;). Because of the dynamic behaviour of an incremental multi-level and parallel system, syntactic structures should not only cooperate but also compete during generation (see 4.3).</Paragraph>
    <Paragraph position="13"> The basic demands 2., 3., 4. are put forward by Kempen's framework for syntactic tree formation \[Kempen87\]. They serve as a theoretical basis for the development of the syntactic formalism proposed in this paper. The other aspects are explicitly mentioned because they are important for incremental multi-level and parallel generation. Before the formalism and the basic operations are described in more detail we briefly introduce POPEL-HOW, the generator in which the fornaalisnl is embedded</Paragraph>
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