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<Paper uid="J99-3004">
  <Title>Interpreting and Generating Indirect Answers</Title>
  <Section position="4" start_page="394" end_page="398" type="intro">
    <SectionTitle>
3. Reversible Knowledge
</SectionTitle>
    <Paragraph position="0"> As shown informally in the previous section, coherence relations can be used to characterize various types of satellites of full answers. Coherence rules, described in Section 3.1, provide sufficient conditions for the mutual plausibility of a coherence relation. During generation, plausibility of a coherence relation is evaluated with respect to the beliefs that R presumes to be shared with Q. During interpretation, the same rules are evaluated with respect to the beliefs Q presumes to be shared with R. Thus, during generation R assumes that a coherence relation that is plausible with respect to his shared beliefs would be plausible to Q as well. That is, Q ought to be able to recognize the implicit relation between the nucleus and satellite.</Paragraph>
    <Paragraph position="1"> However, the generation and interpretation of indirect answers requires additional knowledge. For example, for R's contribution to be recognized as an answer, there must be a discourse expectation (Levinson 1983; Reichman 1985) of an answer. Also, during interpretation, for a particular answer to be licensed by R, the attribution of R's intention to convey that answer must be consistent with Q's beliefs about R's intentions. For example, a putative implicature that p holds would not be licensed if R provides a disclaimer that it is not R's intention to convey that p holds. This and other types of knowledge about full answers is represented as discourse plan operators, described in Section 3.2. In our model, a discourse plan operator captures shared, domain-independent knowledge that is used, along with coherence rules, by 16 This may seem to conflict with the idea in RST that the nucleus, being more essential to the writer's purpose than a satellite, cannot be omitted. However, at least in the case of the coherence relations playing a role in our model, it appears that the nucleus need not be given explicitly when it is inferable in the discourse context.  Computational Linguistics Volume 25, Number 3 It is mutually plausible to the agent that (cr-obstacle q p) holds, where q is the proposition that a state Sq does not hold during time period tq, and p is the proposition that an event e v does not occur during time period t v, if the agent believes it to be mutually believed that Sq is a precondition of a typical plan for doing ev, and that tq is before or includes tv, unless it is mutually believed that sq does hold during tq, or that ep does occur during tp.</Paragraph>
    <Paragraph position="2"> It is mutually plausible to the agent that (cr-obstacle q p) holds, where q is the proposition that a state sq holds during time period tq, and p is the proposition that a state sv does not hold during time period tv, if the agent believes it to be nmtually believed that 8q typically prevents sp, and that tq is before or includes tv, unless it is mutually believed that Sq does not hold during lq, or that s v does hold during t v.</Paragraph>
    <Paragraph position="3"> Figure 1 Glosses of two coherence rules for cr-obstacle.</Paragraph>
    <Paragraph position="4"> the generation component to construct a discourse plan for a full answer. Interpretation is modeled as inference of R's discourse plan from R's response using the same set of discourse plan operators and coherence rules. Inference of R's discourse plan can account for how Q derives an implicated answer, since a discourse plan explicitly represents the relationship of R's communicative acts to R's beliefs and intentions. Together, the coherence rules and discourse plan operators described in this section make up the reversible pragmatic knowledge, i.e., pragmatic knowledge used by both the generation and interpretation components, of the model. Other pragmatic knowledge, used only by the generation process to constrain content planning, is presented in Section 5.</Paragraph>
    <Section position="1" start_page="395" end_page="396" type="sub_section">
      <SectionTitle>
3.1 Coherence Rules
</SectionTitle>
      <Paragraph position="0"> Coherence rules specify sufficient conditions for the plausibility to an agent with respect to the agent's shared beliefs (which we hereafter refer to as the mutual plausibility) of a relational proposition (CR q p), where CR is a coherence relation and q and p are propositions. (Thus, if the relational proposition is plausible to R with respect to the beliefs that R presumes to be shared with Q, R assumes that it would be plausible to Q, too.) To give some examples, glosses of some rules for the coherence relation, which we refer to as cr-obstacle are given in Figure 1.17 The first rule characterizes a subclass of cr-obstacle, illustrated in (9), relating the nonoccurrence of an agent's volitional action (reported in (9)ii) to the failure of a precondition (reported in (9)iii) of a potential plan for doing the action.</Paragraph>
      <Paragraph position="1">  (9) i. Q: Are you going to campus tonight? ii. R: No.</Paragraph>
      <Paragraph position="2"> iii. My car's not running.</Paragraph>
      <Paragraph position="3"> 17 For readability, we have omitted the prefix cr- in Tables 1 and 2.</Paragraph>
      <Paragraph position="4">  Green and Carberry Indirect Answers In other words, it is mutually plausible to an agent that the propositions conveyed in (9)iii and (9)ii are related by cr-obstacle, provided that the agent has a shared belief that a typical plan for R to go to campus has a precondition that R's car is running. The second rule in Figure 1 characterizes another subclass of cr-obstacle, illustrated in (10), relating the failure of one condition (reported in (10)i) to the satisfaction of another condition (reported in (10)ii).</Paragraph>
      <Paragraph position="5"> (10) i. R: My car's not running.</Paragraph>
      <Paragraph position="6"> ii. The timing belt is broken.</Paragraph>
      <Paragraph position="7"> In other words, it is mutually plausible to an agent that the propositions conveyed in (10)ii and (10)i are related by cr-obstacle, provided that the agent has a shared belief that having a broken timing belt typically prevents a car from running.</Paragraph>
      <Paragraph position="8"> Coherence rules are evaluated with respect to an agent's shared beliefs. Coherence rules and the agent's beliefs are encoded as Horn clauses in the implementation of our model. The sources of an agent's shared beliefs include: terminological knowledge: e.g., that driving a car is a type of action, domain knowledge, including -- domain planning knowledge: e.g., that a subaction of a typical plan to go to campus is to drive to campus, and that a typical plan for driving a car has a precondition that the car is running, -- other domain knowledge: e.g., that a broken timing belt typically prevents a car from running, and * the discourse context: e.g., that R has asserted that R's car is not running.</Paragraph>
    </Section>
    <Section position="2" start_page="396" end_page="398" type="sub_section">
      <SectionTitle>
3.2 Discourse Plan Operators
</SectionTitle>
      <Paragraph position="0"> The discourse plan operators provided in the model encode generic programs for expressing full answers (and subcomponents of full answers). TM For example, the discourse plan operators for constructing full yes (Answer-yes) and full no (Answer-no) answers are shown in Figure 2.19 The first line of a discourse plan operator, its header, e.g., (Answer-yes s h ?p), gives the type of discourse action, the participants (s denotes the speaker and h the hearer), and a propositional variable. (Propositional variables are denoted by symbols prefixed with &amp;quot;?&amp;quot;.) In top-level operators such as Answer-yes and Answer-no, the header variable would be instantiated with the questioned proposition. Applicability conditions, when instantiated, specify necessary conditions for appropriate use of a discourse plan operator. 2deg For example, the first applicability condition of Answer-yes and Answer-no requires the speaker and hearer to share the discourse expectation that the speaker will inform the hearer of the speaker's evaluation of the truth of the questioned proposition p. Present in each of the five top-level answer operators, this particular applicability condition restricts the use of these operators to contexts where an answer is expected, 18 The particular formalism we adopted to encode the operators was chosen to provide a concise and perspicuous organization of the knowledge required for our interpretation and generation components. We make no further claims about the formalism itself. 19 There are three other &amp;quot;top-level&amp;quot; operators in the model for expressing the remaining types of full answers illustrated in Table 1. 20 In general, an applicability condition is a condition that must hold for a plan operator to be invoked, but that a planner will not attempt to bring about (Carberry 1990).</Paragraph>
      <Paragraph position="1">  Discourse plan operators for yes and no answers.</Paragraph>
      <Paragraph position="2"> and is needed to account for the hearer's attempt to interpret a response as an answer, even when it is not a direct answer, m The second applicability condition of the top-level operators requires the speaker to hold the evaluation of p to be conveyed; e.g., in Answer-no it requires that the speaker believe that p is false. The primary goals of a discourse plan specify the discourse goals that the speaker intends for the hearer to recognize, n For example, the primary goal of Answer-yes can be glossed as the goal that Q will accept the yes answer, at least for the purposes of the conversation. 23 The nucleus and satellites of a discourse plan describe primitive or nonprimitive acts to be performed to achieve the primary goals of the plan. 24 Inform is a primitive act that can be realized directly. The nonprimitive acts are defined by discourse plan operators themselves. (Thus, a discourse plan may have a hierarchical structure.) A full answer may contain zero, one, or more instances of each type of satellite, and the default (but not required) order of nucleus and satellites in a full answer is the order given in the corresponding operator.</Paragraph>
      <Paragraph position="3"> Consider the Use-elaboration and Use-obstacle discourse plan operators, shown in Figure 3, describing possible satellites of Answer-yes and Answer-no, respectively. All satellite operators include a second propositional variable referred to as the existential 21 Without recourse to the notion of discourse expectation, it is difficult to account for the interpretation in (9)iii of My car's not running as The speaker is not going to campus tonight, while blocking interpretations such as The speaker will rent a car. Note that the latter interpretation may be licensed when the discourse expectation is that R will provide an answer to Are you going to rent a car? In general, discourse expectations provide a contextual constraint on what inferences are licensed by the speaker. (Similarly, it has been argued that scalar implicatures depend on the existence of a salient partially ordered set in the discourse context; see Section 4.3.) For a discussion of the overall role of discourse expectations in our model, see Section 4.2. One might argue that this type of applicability condition limits the generality of the operators and thus, could lead to a proliferation of context-specific operators, which would result in inefficient processing. First, we are not claiming that all discourse operators require this type of applicability condition, only those operators characterizing discourse-expectation-motivated units of discourse. Second, with an indexing scheme sensitive to discourse expectations, this would not necessarily lead to efficiency problems.</Paragraph>
      <Paragraph position="4"> 22 We refer to these as primary to distinguish them from other discourse goals the speaker may have but that he does not necessarily intend for the hearer to recognize.</Paragraph>
      <Paragraph position="5"> 23 During interpretation (see Section 4.1), in order for the implicature to be licensed, the applicability conditions and primary goals of any plan ascribed to R must be consistent with Q's beliefs about R's beliefs and goals. Thus, applicability conditions and primary goals play an important role in canceling spurious putative implicatures. 24 The discourse plan operators in our model are not intended to describe all acts that may accompany a direct answer. For example, the model does not address the generation of parts of the response, such as repetition or restatement, which entail the answer.</Paragraph>
      <Paragraph position="6">  variable. For example, (9)ii-(9)iii could be described by a plan constructed from an Answer-no discourse plan operator whose header variable is instantiated with the proposition p that R is going to campus tonight, and which has a satellite constructed from a Use-obstacle discourse plan operator whose header variable is instantiated with (not p), the proposition that R is not going to campus tonight, and whose existential variable q is instantiated with the proposition that R's car is not running.</Paragraph>
      <Paragraph position="7"> In general, each satellite operator in our model has applicability conditions and primary goals analogous to those shown in Figure 3. (Each satellite operator has a name of the form, Use-CR, where CR is the name of a coherence relation.) The first applicability condition of a satellite operator, Use-CR, requires that the speaker believes that the relational proposition (CR q p) holds for propositions q and p instantiating the existential variable and header variable, respectively. The second applicability condition requires that, given the beliefs that the speaker presumes to be shared with the hearer, this relational proposition is plausible. (Mutual plausibility is evaluated using the coherence rules described in Section 3.1.) The primary goal of a satellite operator can be glossed as the goal that the hearer will accept the relational proposition.</Paragraph>
    </Section>
  </Section>
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