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<Paper uid="W99-0111">
  <Title>Reference-based Discourse Structure for Reference Resolution</Title>
  <Section position="8" start_page="456" end_page="456" type="concl">
    <SectionTitle>
6 Conclusion
</SectionTitle>
    <Paragraph position="0"> We have described aunified approach to reference resolution and discourse structure which is implemented in a system of language understanding. As well as exploiting discourse structure for reference resolution, we explore the extent to which referential information can in turn be exploited to provide the discourse structure to be used by subsequent reference resolution. The strengths and weaknesses of the different referential cues investigated have already been discussed in the previous section. Here we highlight some of the other issues raised.</Paragraph>
    <Paragraph position="1"> What, if any, is the role of discourse structure in the dereferencing of short, non-unique Nl~s referring to familiar referents? We have argued that the differences between the felicity conditions for the use of pronouns and non-pronominalised NPs suggest that the former are dependent on discourse structure but the latter are not. This has further implications for our account in that it means we in turn exploit pronominalised references, but not non-pronominalised references, in building discourse structure. The different Uses of pronouns and non-pronominalised NPs have also been noted by (Hitseman and Poesio, 1995), although with a view to reference generation.</Paragraph>
    <Paragraph position="2"> While space precludes a full diso,_~_'on of their approach, it is worth wmmarki~ the diffe~nce between their approach and ours here. They assume that discourse structm~ is used for the resolution of all referring expreui'ons, but that, while pronouns can only refcr to an entity which w~ at one point the Most Salient gntity ~s in the discourse, definite descriptions can refer to any entity on the discourse stack. We assume that Mu~ structure is used only for the resolution of pmnominalised refen~ expressions. Related to this, we exploit the fact that items which c~n-be pronomlnalised normally show/d be, in order to impose further structure on the discourse. If a short NP is used where a pronoun would have been interpretable, we assume that the current utterance should not be attached, either as a daughter or a sister, to the most recent node. Failing to ssThis notion k closely rdated to that d the center.</Paragraph>
    <Paragraph position="3"> take advantage of an attachment clue, such as the use of pronominalisation of theme, is seen a positive instruction not to attach at a certain point, rather than just as a lack of information.</Paragraph>
    <Paragraph position="4"> Our observation of the effects of the choice of referring expression, whether pronominalised or not, on discourse structure raised a number of issues which need to be explored further. The first of these is the need to make a distinction between the effects of reyeren~s and re/er~ng ezpressions on discourse structure, since the choice of a different referring expression for the same referent may convey a different discourse move. Closely related to this, there is the need to consider how referential continuity may be indicated where pronominalisation is not an option, as, for instance, by the use of bridging descriptions linking non-identical but closely related referents.</Paragraph>
    <Paragraph position="5"> Finally there is the need to consider whether attachment choices may be better explained in terms of negative constraints on attachment rather than (or in addition to) the attachment preferences we have assumed here.</Paragraph>
    <Paragraph position="6"> A further observation we made was that discourse structure constrains temporal structure, and so temporal reference, in the same way that temporal structure constrains discourse structure. While reference time may provide us with a reliable cue to discourse structure, if our preferred reference time turns out to be wrong then we also get the discourse structure wrong. Similarly, if we had operated attachment preferences before reference resolution, the converse would have held. Either way, we are faced with the dilemma that it is not until we.have bo~ a potential set of referents and a potential attachment site that inference can be used to determ;ne whether any plausible discourse relations hold giveh the choices we have made.</Paragraph>
  </Section>
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