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<Paper uid="P98-2211">
  <Title>tappe, Coherence in Spoken Discourse*</Title>
  <Section position="4" start_page="1295" end_page="1295" type="metho">
    <SectionTitle>
4 Discourse grammar
</SectionTitle>
    <Paragraph position="0"/>
    <Section position="1" start_page="1295" end_page="1295" type="sub_section">
      <SectionTitle>
4.1 Tree descriptions
</SectionTitle>
      <Paragraph position="0"> A definition of TDG is given by Kallmeyer (1996) who introduces tree descriptions consisting of constraints for finite labelled trees. A dominance relation (&lt;~*) between node labels indicates that these two labels can be equated or have a path of arbitrary length inserted between them. The second relation between nodes is the parent relation (&lt;~) which is irreflexive, asymmetric and intransitive.</Paragraph>
      <Paragraph position="1"> The tree's root node D labelled kl in figure 2, for example, dominates another node labelled k2.</Paragraph>
      <Paragraph position="2"> According to the definition of &lt;~* these two nodes may be equal or an arbitrary number of other nodes may be in between them. An adjoining operation</Paragraph>
      <Paragraph position="4"> is easily defined because of this property. Further tree descriptions can be inserted between such nodes. The descriptions which are, formally speaking, negation-free formulae of constraints on the nodes, are conjoined. The nodes where the adjunction takes place are set to equal.i ~Figure 3 shows an example.</Paragraph>
    </Section>
    <Section position="2" start_page="1295" end_page="1295" type="sub_section">
      <SectionTitle>
4.2 A flexible discourse grammar
</SectionTitle>
      <Paragraph position="0"> According to Schilder (1997), feature value structures are added to the tree logic in order to enrich it with rhetorical relations and further discourse information. One non-terminal symbol is used for the D(iscourse) segments, whereas the terminals are the S(entences).</Paragraph>
      <Paragraph position="1"> Two features are added to the tree description to encode the semantic content of the sentence and the 'topic' information expressed in a discourse. Firstly, S gets associated with the meaning of a sentence via a feature CONT(ENT) containing all discourse referents and the conditions imposed on them. 2 Secondly, a feature PROMI(NENT) is added that is used to define the notion of openness within a discourse. This feature refects the fact that one situation described by an utterance (e.g. situation el described by U1) is subordinated by another one when combined via a rhetorical relation. It furthermore exhibits the restriction of the further utterances to the right frontier of the discourse tree (cf. (Webber, 1991)).</Paragraph>
      <Paragraph position="2"> For the discourse structure two types of tree descriptions have to be distinguished. One tree structure allows attachment on two levels of the right frontier of the tree. This tree is called subordinated tree and the structure is schematically indicated in figure 2. The other one is a subordinating structure that is triggered by discourse relations such as narration or result. Further attachment is only possible at the last uttered sentence. 3</Paragraph>
    </Section>
  </Section>
  <Section position="5" start_page="1295" end_page="1296" type="metho">
    <SectionTitle>
5 Formalisation
</SectionTitle>
    <Paragraph position="0"> The discourse structure obtained for the first three sentences of the example text is reflected in figure  3. At first an elaboration relation is established between (U1) and (U2). The imposed discourse struc- null ture (i.e. a subordinated tree as in figure 2) allows attachment at two levels. Note furthermore that the elaboration relation holds between the mental state of the producer (i.e. I already know what this is all about) and the description of what is happening on the screen. 4 (U3) is connected with (U2) via narration. The adjunction operation in figure 3 shows how the</Paragraph>
    <Paragraph position="2"> newly generated sentence is incorporated in the current discourse structure.</Paragraph>
    <Paragraph position="3"> Although the production took place under a certain amount of pressure, the right frontier principle was never violated. The speaker never went back or made anaphoric references to discourse referents being behind this frontier.</Paragraph>
    <Paragraph position="4"> Having demonstrated how the production of the discourse structure can be formally described for the first three utterances, we now want to focus on a particularly interesting problem exhibited by the sequence (U4) to (U6). This sequence contains rhetorical questions, which describe the ongoing planning process of the speaker. 5 The sequence starts with an expectation (i.e. (U4)) the subject utters. Again the proposition expressed is related to the mental state of the speaker. Interestingly enough, he has to return to the top level of the discourse tree and continue from there. Consequently, the discourse segment containing (U2) and (U3) is 'cut off' and not available for further attachment.</Paragraph>
    <Paragraph position="5"> Embedded within the expectation is an utterance describing the ongoing planning and searching process. The verbalised questions reflect the request to the mental lexicon and the mental map the subject has got of this area.</Paragraph>
    <Paragraph position="6"> The discourse grammar consequently has to be SNote that such a sequence would never be found in a written text.</Paragraph>
    <Paragraph position="7"> extended in order to maintain a coherent discourse structure for the modelling of the producer. Thus rhetorical relations describing planning processes are introduced. With these, the discourse grammar becomes capable of representing a coherent discourse structure for the spoken language despite the fact that the entire discourse segment does not seem as coherent as written text.</Paragraph>
    <Paragraph position="8"> Figure 4 contains the discourse structure after the search for the street name has come to an end. One rhetorical relation introduced is p(lan)_comment which describes the ongoing planning process. It also involves a search for the correct word in the lexicon. The rhetorical quest(ion) is asked whether the correct word has been chosen and this question answered by the subject. The summarising yes, isn't it (i.e. (U6b)) ends the search process and closes the discourse structure at the right frontier.</Paragraph>
    <Paragraph position="9"> Interestingly enough, the clue given by the discourse marker but uttered in (U7) is absolutely essential. The speaker indicates with this marker that he wants to return to the top level of the discourse tree and to add a contrast relation to the expectation. The construction of the discourse structure continues therefore at the top level of the tree in figure 4.</Paragraph>
  </Section>
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