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<Paper uid="C96-2196">
  <Title>Parsing Plans Situation-Dependently in Dialogues</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> Dialogue understanding requires plan recognition.</Paragraph>
    <Paragraph position="1"> Many plan inference models have thus been proposed. As an approach to the computation of plan recognition from observed actions, plan parsing hms been proposed by Sidner (1985) and formalized by Vilain (1990). A typical plan recipe for an action includes a sequence of subactions as its decomposition, so interpreting an action sequence in terms of plans can be seen as parsing in which observed actions correspond to lexieal tokens and plan recipes correspond to grammatical rules.</Paragraph>
    <Paragraph position="2"> Previous plan parsing methods, however, are insufficient for dialogue understanding since they do not handle the effects and preconditions of actions. These effects and preconditions are of crucial importance in reasoning about what the agent intends to do and what she presupposes. More concretely, without treating them, it is impossible (a) to describe actions in terms of their effects, (b) to capture the relationship between an action and another action that satisfies the former's preconditions to enable it, and (c) to interpret actions in a manner dependent on the dialogue state.</Paragraph>
    <Paragraph position="3"> To solve these problems, we have developed a plan parsing method that can handle the effects and preconditions of actions and that parses plans in a manner dependent on dialogue state changes, especially on the mental state changes of dialogue participants caused by dialogue utterances.</Paragraph>
    <Paragraph position="4"> This method, in particular, makes (a) (c) possible. The method is based on active chart parsing and uses augmented edge structures to keep state information locally and time map management (Dean and McDermott, 1987) to deal with state changes. The method is implemented in Sicstus Prolog and is applied to a dialogue understanding system (Shimazu et al., 1994).</Paragraph>
  </Section>
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