<?xml version="1.0" standalone="yes"?>
<Paper uid="W94-0322">
  <Title>Generating Indirect Answers to Yes-No Questions</Title>
  <Section position="3" start_page="189" end_page="190" type="metho">
    <SectionTitle>
2. Pragmatic Knowledge
</SectionTitle>
    <Paragraph position="0"> A language user's knowledge of how to answer a Yes-No question appropriately can be described by a set of discourse plan operators representing full answers.</Paragraph>
    <Paragraph position="1"> A full answer consists of a direct answer (which we call the nucleus) and, possibly, extra relevant information (satellites). An indirect answer can be modeled as the result of R's providing one or more satellites without expressing the nucleus explicitly. Due to coherence constraints holding between nucleus and satellite, Q may infer R's discourse plan by inferring which coherence relation between an indirect answer and a candidate direct answer is plausible. The coherence relations are similar to the subject-matter relations of Rhetorical Structure Theory (RST) \[MT87\]. ~ The discourse plan operators used to generate and interpret R's response in (1) are given in Figures 1 and 2. Figure 1 depicts the top-level operator for constructing a No answer. To explain our notation, s and h are constants denoting speaker (R) and hearer (Q), respectively. Symbols prefixed with &amp;quot;?&amp;quot; denote propositional variables. The variable in the header of Answer-no will be instantiated with the questioned proposition, e.g. in (1) that R is going shopping. Applicability conditions are necessary conditions for appropriate use of the operator. For example, for a No answer to be appropriate, R must believe that the questioned proposition p is false. Also, an answer is not appropriate unless s and h share the expectation 2The terms nucleus and satellite have been borrowed from RST. Note that according to RST, a property of the nucleus is that its removal results in incoherence. However, in our model, a direct answer may be removed without causing incoherence, provided that it is inferrable from the rest of the response.</Paragraph>
    <Paragraph position="2">  that s will provide s's evaluation of the truth of p, which is denoted as (discourse-expectation (informif s h p)). Primary goals describe the intended effects of the operator. We use (BMB h s p) to denote that h believes it to be mutually believed with s that p \[CM81 \]. In general, the nucleus and satellites of a discourse plan operator describe primitive or non-primitive communicative acts. Our plan formalism allows zero, one, or more occurrences of a satellite in a full answer. The expected (but not required) order of nucleus and satellites is the order they are listed in an operator. (inform s h p) denotes the primitive act of s informing h that p. The satellites in Figure 1 refer to non-primitive acts, described by discourse plan operators which we have defined (one for each coherence relation used in a full answer). For example, Use-obstacle, a satellite of Answer-no, is defined in Figure 2.</Paragraph>
    <Paragraph position="3"> To explain the additional notation in Figure 2, (cr-obstacle q p) denotes that the coherence relation  cr-obstacle holds between q and p.S Thus, the first applicability condition can be glossed as requiring that s believe that the coherence relation holds. In the second applicability condition, (Plausible (cr-obstacle q p)) denotes that_, given what s believes to be mutually believed with h, the coherence relation (cr-obstacle q p) is plausible. In our model, this sort of condition is evaluated using a set of coherence rules based on the relation definitions of RST.</Paragraph>
    <Paragraph position="4"> In summary, we provide our system with a set of discourse plan operators representing shared domain-independent knowledge of the informational content of a full answer. (Operators whose names begin with Answer are top-level Operators.) Table 1 shows the satellites of each top-level discourse plan operator. In addition, we provide a set of coherence rules defining the plausibility of the Coherence relations generated by the satellite discourse plan operators. 4</Paragraph>
  </Section>
  <Section position="4" start_page="190" end_page="193" type="metho">
    <SectionTitle>
3. Stimulus Conditions
</SectionTitle>
    <Paragraph position="0"> Applicability conditions prevent inappropriate use of a discourse plan operator. However, they do not model a speaker's motivation for choosing to provide extra information. A full answer might provide too much information if every satellite whose applicability conditions held were included in the full answer. On the other hand, at the time when he is asked a question, R may not have the primary goals of a potential satellite operator. Thus, a goal-driven approach to selecting satellites would provide insufficient information. To overcome these limitations, we have incorporated stimulus conditions into the discourse plan operators in our  identified the following coherence relations in our corpus: cr-obstacle, cr-possible-cause, cr-possible-obstacle, and crusually. null model. Playing a key role in content determination, stimulus conditions describe conditions motivating a speaker to include a satellite during plan construction. They can be thought of as triggers which give rise to new speaker goals. In order for a satellite to be included, all of its applicability conditions and at least one of its stimulus conditions must be true. While stimulus conditions may be derivative of deeper principles of cooperativity \[Gri75a\] or politeness \[BL78\], they provide a level of precompiled knowledge which reduces the amount of reasoning required for content determination.</Paragraph>
    <Paragraph position="1"> Our methodology for identifying stimulus conditions was to survey linguistic studies (described below), as well as to analyze the possible motivation of the speaker in the examples in our corpus. According to StenstrSm \[Ste84\], the typical motivation for providing extra information is to answer an implicit wh-question. Other reasons cited are to provide an explanation justifying a negative answer, to qualify the answer, or &amp;quot;social reasons&amp;quot;. Levinson \[Lev83\] claims that the presence of an explanation is a structural characteristic of dispreferred responses, such as refusals and unexpected answers. Brown and Levinson \[BL78\] show how various uses of language are motivated by politeness. For example, use of conversational implicature is one strategy for a speaker to avoid doing a face-threatening act, s and as we discuss in section 6, an indirect answer conversationally implicates the direct answer. Hirschberg \[Hir85\] claims that an indirect scalar response, rather than a Yes or No answer, is necessary in certain cases to avoid misleading the hearer.</Paragraph>
    <Paragraph position="2"> In the rest of this section, we describe each of the stimulus conditions used in our system. For each stimulus condition, we give one or more axioms defining sufficient conditions for it. Axioms are encoded as Horn clauses, where symbols prefixed with &amp;quot;?&amp;quot; are variables, all variables are implicitly universally quantified, and the antecedent conditions are implicitly conjoined. The head, i.e. the consequent, of an axiom is a stimulus condition appearing in one or more of our discourse plan operators. Table 2 summarizes which stimulus conditions appear in which operators. For example, the stimulus condition (explanation-indicated s h ?p ?q) occurs in the discourse plan operator shown in Figure 2. Keep in mind that additional constraints on .C/p and ?q arise from the applicability conditions of the operator containing the stimulus condition.</Paragraph>
    <Section position="1" start_page="190" end_page="192" type="sub_section">
      <SectionTitle>
3.1 Definitions
Explanation-indicated
</SectionTitle>
      <Paragraph position="0"> This stimulus condition appears in all of the oper-SThey claim that another strategy is to give the direct answer last, i.e. in our terminology, to give the satellite(s) before the nucleus.</Paragraph>
      <Paragraph position="1">  tors ators for providing causal explanations. For example in (2), 6 which indirectly conveys a No, R gives an explanation of why R won't get a car.</Paragraph>
      <Paragraph position="2"> 2.a. q: actually you'll probably get a car won't you as soon as you get there b. R: can't drive Note that this stimulus condition may contribute to greater dialogue efficiency by anticipating a follow-up request for an explanation. Formally, the condition is defined by the following axiom.</Paragraph>
      <Paragraph position="3"> ((explanation-indicated s h ?p ?q)  &lt;(wbel s (wbel h (not ?p))) (unless (wbel s (accepts-authority h s)))) This may be glossed as, s is motivated to give h an explanation q for p, if s suspects that h suspects that p is not true, unless it is the case that s has reason to believe that h will accept p on s's authority. (wbel agent p), which we usually gloss as agent suspects that p, denotes that agent has some confidence in the belief that p. Note tha, t R may acquire the suspicion that Q doubts that p is true by means of syntactic clues from the Yes-No question, e.g. the form of the question in (2@ Excuse-indicated Although this stimulus condition appears in some of the same causal operators as Explanation-indicated, ~Stenstr6m's (110) it represents a different kind of motivation. A Yes-No question may be interpreted as a prerequest \[Lev83\] for a request, i.e. as an utterance used as a preface to another request. 7 Prerequests are often used to check whether a related request is likely to succeed, or to avoid having to make the other request directly. Thus, a negative answer to a Yes-No question used as a prerequest may be interpreted as a refusal. To soften the refusal, the speaker may give an explanation of the negative answer, as illustrated in (1). Formally, the condition is defined by the following axiom.</Paragraph>
      <Paragraph position="4"> ((excuse-indicated s h (not ?p) ?q)  &lt;(~bel s (prerequest h s (informif s h ?p)))) This may be glossed as, s is motivated to give h an excuse q for (not p), if s suspects that h's request, (informif s h p), is a prerequest. Techniques for interpreting indirect speech acts \[PA80, Hin89\] can be used to determine whether the antecedent holds.</Paragraph>
      <Paragraph position="5"> Answer-ref-indicated This condition appears in Use-Elaboration, illustrated by (3), s and in Use-Contrast, illustrated by  (4) .9 3.a. Q: b.</Paragraph>
      <Paragraph position="6"> c. E: d.</Paragraph>
      <Paragraph position="7"> Did you have a hotel in mind? \[What hotel did you have in mind?\] \[yes\] There's a Holiday Inn right near where I'm working.</Paragraph>
      <Paragraph position="8"> 4.a. Q: You're on that? b. \[Who's on that?\] c. R: no no no d. Dave is.</Paragraph>
      <Paragraph position="9">  In (3), R has interpreted the question in (a) as a prerequest for the wh-question shown in (b). Thus, (d) not only answers the question in (a) but also the anticipated wh-question in (b). Similarly in (4), R may interpret the question in (a) as a prerequest for the wh-question in (b), and so gives (d) to provide an answer to both (a) and (b). Formally, the condition is defined by the following axiom.</Paragraph>
      <Paragraph position="10"> ((Answer-re:f-indicated s h ?p ?q)  &lt;(wbel s (want h (knowref h ?t ?q))) This may be glossed as, s is motivated to provide h with q, if s suspects that h wants to know the referent Tin analyses of discourse based on speech act theory, e.g. \[PA80\], a question such as (lb) might be described as an indirect request. However, since it may be responded to appropriately by a Yes or No, it is included in the class of Yes-No questions addressed in our model.</Paragraph>
      <Paragraph position="11"> SAmerican Express tape 1 9StenstrSm's (102). Note that a No answer may be conversationally implicated by use of (4d) alone.</Paragraph>
    </Section>
    <Section position="2" start_page="192" end_page="193" type="sub_section">
      <SectionTitle>
7th International Generation Workshop * Kennebunkport, Maine * June 21-24, 1994
</SectionTitle>
      <Paragraph position="0"> of a term t in q. As in Excuse-indicated, techniques for interpreting indirect speech acts can be used to determine if the antecedent holds.</Paragraph>
      <Paragraph position="1"> Substitute-indicated This condition appears in Use-contrast, illustrated by (5).</Paragraph>
      <Paragraph position="2"> S.a.Q: Do you have Verdi's Otello or Aida? b.R: \[no\] c. We have Rigoletto.</Paragraph>
      <Paragraph position="3"> Although Q may not have intended to use (5a) as a prerequest for the qu#stion What Verdi operas do you have.C, R suspects that the answer to this wh-question might be helpful to Q. Formally, the condition is defined by the following axiom.</Paragraph>
      <Paragraph position="4"> ((Substitute-indicated s h ?p ?q)  &lt;(wbel s (need h (knowref h ?t ?q)))) This may be glossed as, s is motivated to provide h with q, if s suspects that it would be helpful for h to know the referent of a term t in q. The antecedent would hold wl~enever obstacle detection techniques \[AP80\] determine that h's not knowing the referent of t is an obstacle to an inferred plan of h's. However, note that not all helpful responses, in the sense described in lAPS0\], can be used as indirect answers. For example, even if the clerk (R) at the music store believes that Q's not knowing the closing time could be an obstacle to Q's buying a recording, tl/e response (6), given instead of (5c), would not convey (55) since it cannot be coherently related to (55).</Paragraph>
      <Paragraph position="5"> 6. R: We close at 5:30 tonight.</Paragraph>
      <Paragraph position="6"> Clarify-concept-indicated This stimulus condition appears Use-elaboration, as illustrated by (7)3 0 in 7. Q: Do you have a pet? R: We have a turtle.</Paragraph>
      <Paragraph position="7"> In (7), R was motivated to elaborate on the type of pet they have, since turtles are not prototypical pets. Formally, the condition is defined by the following axiom. null  This may be glossed as, s is motivated to clarify p to h with q, if p has a concept c, and q provides an atypical instance of c. Stereotypical knowledge is used to evaluate the second antecedent.</Paragraph>
      <Paragraph position="8"> 1deg Hirschberg's (177).</Paragraph>
      <Paragraph position="9"> Clarify-condition-indicated This stimulus condition appears in the operator Use-condition, as illustrated by (8). 11 8.a. Q: Um let me can I make the reservation and change it by tomorrow b. R: \[yes\] c. if it's still available.</Paragraph>
      <Paragraph position="10"> In (8), a truthful Yes answer depends on the truth of (c). Formally, the stimulus condition is defined by the following axioms.</Paragraph>
      <Paragraph position="11">  These may be glossed as, s is motivated to clarify a condition q for p to h 1) if s doesn't know if q holds, or 2) if s suspects that q does not hold.</Paragraph>
      <Paragraph position="12"> Clarify-extent-indicated This stimulus condition appears in Use-contrast, as illustrated by (9). 12 9.a. Q: Have you gotten the letters yet? b. R: I've gotten the letter from X.</Paragraph>
      <Paragraph position="13"> On the strict interpretation of (9a), Q is asking whether R has gotten allof the letters, but on a looser interpretation, Q is asking if R has gotten any of the letters. Then, if R has gotten some but not all of the letters, a Yes would be untruthful. However, if Q is speaking loosely, then a No might lead Q to erroneously conclude that R has not gotten any of the letters. R's answer circumvents this problem, by conveying the extent to which the questioned proposition (on the strict interpretation) is true. Formally, the condition is defined by the following axioms.</Paragraph>
    </Section>
    <Section position="3" start_page="193" end_page="193" type="sub_section">
      <SectionTitle>
7th International Generation Workshop * Kennebunkport, Maine * June 21-24, 1994
</SectionTitle>
      <Paragraph position="0"> holds, and s believes that q is the highest expression alternative to p that does hold. According to I-Iirschberg \[Hir85\] (following Gazdar), sentences Pi and pj (representing the propositional content of two utterances) are expression alternatives if they are the same except for having comparable components ei and e j, respectively. Further, Hirschberg claims that in a particular discourse context, there may be a partial ordering of values which the discourse participants mutually believe to be salient, and that the ranking of ei and ej in this ordering can be used to describe the ranking of pi and pj. In the above example, (9b) is a realization of the highest true expression alternative to the questioned proposition, p, i.e. the proposition that R has gotten all the letters, is Appeasement-indicated This stimulus condition appears in Use-contrast, as illustrated by (10). 14 10.a. Q: Did you manage to read that section I gave you? b. R: I've read the first couple of pages.</Paragraph>
      <Paragraph position="1"> In (10), R conveys that there is some (though not much) truth to the questioned proposition in an effort to soften his answer. Note that more than one stimulus condition may motivate R to include the same satellite. E.g., in (10), R may have been motivated also by clarify-eztent-indicated, which was described above. However, it is possible to provide a context for (9) where appeasement-indicated holds but not clarifyextent-indicated, or a context where the converse is true. Formally, the condition is defined by the following axiom.</Paragraph>
      <Paragraph position="2"> ((appeasement-indicated s h (not ?p) ?q) &lt;(wbel s (undesirable h (not ?p))) (wbel s (desirable h ?q)))</Paragraph>
      <Paragraph position="4"> (wbel s (undesirable h (not ?p))) (wbel s (desirable h ?q))) This may be glossed as, s is motivated to appease h with q for p not holding or only being partly true, if s suspects that (not p) is undesirable to h but that q is desirable to h. The antecedents to this rule would be evaluated using heuristic rules and stereotypical and specific knowledge about h's desires. For example, two 13Recall that additional constraints on p and q arise from the applicability conditions of operators containing this stimulus condition, namely Use-contrastin this case. Thus, another constraint is that it is plausible that cr-contrast holds. Note that cr-contrast also is defined in terms of such an ordering.</Paragraph>
      <Paragraph position="5"> a4 Hirchberg's (56) heuristics of rational agency that might lead to beliefs about h's desires are 1) if an agent wants you to perform an action A, then your failure to perform A may be undesirable to the agent, and 2) if an agent wants you to do A, then it is desirable to the agent that you perform a part of A.</Paragraph>
    </Section>
  </Section>
  <Section position="5" start_page="193" end_page="196" type="metho">
    <SectionTitle>
4. Generation Algorithm
</SectionTitle>
    <Paragraph position="0"> The inputs to generation in our model consist of 1) a set of discourse plan operators augmented with stimulus conditions, 2) a set of coherence rules, 3) R's beliefs, and 4) the discourse expectation that R will provide R's evaluation of the truth of the questioned proposition p. The output of the generation algorithm is a discourse plan which can be realized by a tactical generation component \[McK85\]. We assume that when answer generation begins, the speaker's only goal is to satisfy the above discourse expectation35 Our answer generation algorithm has two phases.</Paragraph>
    <Paragraph position="1"> In the first phase, content planning, the generator creates a discourse plan for a full answer, i.e., a direct answer and extra appropriate information, e.g. (lc) given explicitly, followed by (ld) : (le). In the second phase, plan pruning, the generator determines which propositions of the planned full answer do not need to be explicitly stated. For example, given an appropriate model of R's beliefs, our system generates a plan for asserting only the proposition conveyed in (le) as an answer to (la) - (lb)36 Note that an advantage of our approach is that, even when it is not possible to omit the direct answer, a full answer is generated.</Paragraph>
    <Section position="1" start_page="193" end_page="194" type="sub_section">
      <SectionTitle>
4.1 Content planning
</SectionTitle>
      <Paragraph position="0"> Content planning is performed by top-down expansion of an answer discourse plan operator. The process begins by instantiating each of the operators with the questioned proposition until one is found such that its applicability conditions hold. Next, this (top-level) answer operator is expanded. A discourse plan operator is expanded by deciding which of its satellites to include in the full answer and expanding each of  stantiated operator hold.</Paragraph>
      <Paragraph position="1"> Note that testing of applicability conditions and stimulus conditions (including searching for an approaSThe plan that is output by our algorithm specifies an ordering of discourse acts based upon the ordering of coherence relations specified in the discourse plan operators. However, to model a speaker who has more than the above initial goal, reordering may be required.</Paragraph>
      <Paragraph position="2"> 16The tactical component must choose an appropriate referring expression to refer to R's car's timing belt, depending on whether (ld) is omitted.</Paragraph>
    </Section>
    <Section position="2" start_page="194" end_page="194" type="sub_section">
      <SectionTitle>
7th International Generation Workshop * Kennebunkport, Maine * June 21-24, 1994
</SectionTitle>
      <Paragraph position="0"> priate instantiation of the existential variable) is handled by a theorem prover. Our model of R's beliefs, represented as a set of Horn clauses, includes 1) general world knowledge presumably shared with Q, 2) knowledge about the preceding discourse, and 3) R's beliefs (including &amp;quot;weaker be~liefs&amp;quot; encoded using the wbel operator) about Q's beliefs. Much of the shared world knowledge needed to evaluate our coherence rules consists of knowledge from domain plan operators.</Paragraph>
    </Section>
    <Section position="3" start_page="194" end_page="194" type="sub_section">
      <SectionTitle>
4.2 Plan pruning
</SectionTitle>
      <Paragraph position="0"> The output of the ~ontent planning phase, an expanded discourse plan representing a full answer, is the input to the plan pruning phase of generation. The expanded plan is represented as a tree of discourse acts.</Paragraph>
      <Paragraph position="1"> The goal of this phase is to make the response more concise, 17 i.e., to determine which of the planned acts can be omitted while lstill allowing Q to infer the full discourse plan. To do this, the generator considers each of the acts in the frontier of the tree from right to left. (This ensures that a satellite is considered before its related nucleus.) The generator creates a trial plan consisting of the original plan minus the nodes pruned so far and minus the current node. Then, using the interpretation imodule, the generator simulates Q's interpretation of a response containing the information that would be given explicitly according to the trial plan. If Q couldlinfer the full plan (as the most preferred interpretation), then the current node can be pruned. Otherwise, itlis left in the plan and the next node is considered.</Paragraph>
      <Paragraph position="2"> For example, consider Figure 3 as we illustrate the possible effect of pruni;ng on a full discourse plan. The leaf nodes, representing discourse acts, are numbered 1 - 8. Arcs labelled N and S lead to a nucleus or satellite, respectively. Node 8 corresponds to the direct answer.</Paragraph>
      <Paragraph position="3"> Plan pruning would process the nodes in order from 1 to 8. The maximal set :of nodes which could be pruned in Figure 3 is the set containing 2, 3, 4, 7, and 8. That is, nodes 2 - 4 might be inferrable from 1, node 7 from 5 or 6, and node 8 from 4 or 7. In the event that it is determined that no node can be pruned, the full plan would be output. Not:e that our interpretation algorithm (described in \[GC94\]) performs a subprocess, hypothesis generation, to recognize missing propositions other than the direct answer, i.e. the propositions at nodes 2, 3, 4, and 7.</Paragraph>
    </Section>
    <Section position="4" start_page="194" end_page="196" type="sub_section">
      <SectionTitle>
4.3 Example
</SectionTitle>
      <Paragraph position="0"> This section gives an ~example of how the discourse plan shown in Figure 4 would be generated, corresponding to the answer shown in (11), where only (lld) is explicitly stated.</Paragraph>
      <Paragraph position="1"> lrConciseness is not the only possible motive for omitting the direct answer. As mentioned in section 3, an indirect answer may be used to avoid performing a face-threatening act.</Paragraph>
      <Paragraph position="2">  c. \[The baby sitter could not sit.\] d. The baby sitter was sick.</Paragraph>
      <Paragraph position="3"> In phase one, the generator creates a discourse plan for a full answer. First, it must decide which top-level discourse plan operator to expand. Each of the top-level operators would be instantiated with the questioned proposition, (occur (go-party s) past). The top-level operators would be instantiated and tested until one was found whose applicability conditions held, namely, Answer-no. Next, the generator must decide which of the satellites of Answer-no to include. So, it would search for all propositions q such that, when the existential variable ?q of a satellite is instantiated with q, then all of the satellite's applicability conditions and at least one of its stimulus conditions hold. In this example, the proposition (not (in-state (can-sit sitter) past)) would be found to satisfy the applicability conditions and the explanation-indicated stimulus condition of Use.obstacle. (Note that this proposition might be chosen because the model of R's beliefs includes a domain plan operator for going to a party, with a pre-condition that the agent's sitter can baby sit.) Next, this instantiation of Use-obstacle must be expanded, i.e., the generator must decide which of the satellites of Use-obstacle to include. By a procedure similar to that described above, it would decide to include (another) Use-obstacle, where the existential variable is instantiated with the proposition (in-state (sick sitter) past). (Note that this proposition might be chosen because the model of R,'s beliefs includes the shared belief that being sick typically renders a sitter unfit to perform his or her duties.) The stimulus condition explanation-indicated might be the motivation for including this satellite. (For example, suppose it is shared knowledge that R's baby sitter is rarely unavailable.) If further attempts to expand the plan were unsuccessful, then the full plan would contain the discourse acts shown in Figure 4. (The acts in the plan are labelled (b) - (d) corresponding to the part of (11) they represent.) Note that if this plan were output  without undergoing plan-pruning, it would be realized as a direct answer (llb), and extra information (llc) -(lld).</Paragraph>
      <Paragraph position="4"> In phase two, plan-pruning, the generator's over-all goal is to make the answer as concise as possible. Therefore, it would consider the primitive acts specified in the full plan in the order (d), (c), (b). Since (d) is not inferrable from any other act, it would be automatically retained in the plan. Next, a trial plan with (c) pruned from it would be considered. Interpretation of the response consisting of (b) and (d) is simulated. Since the full plan could be inferred, (c) would be pruned. Next, another trial plan would be considered, where, in addition to (c), (b) has been pruned. Since the full plan could be inferred from the resulting trial response consisting of just (d), (b) would be pruned too. Thus, the output of phase two would be the plan shown in Figure 4 with (b) and (c) marked as pruned.</Paragraph>
      <Paragraph position="5"> 5. Other factors in generation The primary focus of our research has been on 1) use of pragmatic knowledge which is common to both the interpretation and generation of indirect answers, i.e. the so-called reversible knowledge, and 2) identifying stimulus conditions. In this section, we give a brief description of some other factors in generating indirect answers.</Paragraph>
      <Paragraph position="6"> First, note that we made the simplifying assumption that R's only initial goal is to answer the question (accurately, efficiently, and politely). However, it is possible for R to construct an indirect answer which satisfies multiple initial goals of R. For example, R might have decided to give the answer in (12c) not only as an explanation, but also as background for R's request in (12d).</Paragraph>
      <Paragraph position="7"> 12.a. q: Are you going to the lecture? b. R: \[no\] c. I have to get my brakes fixed.</Paragraph>
      <Paragraph position="8"> d. Can you recommend a garage near c ampus ? Second, stylistic goals may affect the generation of indirect answers. In addition to affecting syntactic and lexical aspects of discourse \[Hovg0, MG91, DH93\], stylistic goals may affect which and how much information is given. For example, elaboration can be used to make an answer more lively, as shown in (13).</Paragraph>
      <Paragraph position="9"> 13,a. Q: Do you have a car? b. R: \[yes\] c. I bought a British-racing-green Austin-Healey 3000 last week.</Paragraph>
      <Paragraph position="10"> Also, extra information that entails the direct answer (e.g. repetitions and generalizations) may be given for emphasis. Further, a stylistic goal of terseness may override the stimulus conditions we have provided for indirect answers. For example, depending upon the degree of Q's interest in R's affairs, the indirect answer in (14c) - (14f) may provide Q with more than Q cares to know.</Paragraph>
      <Paragraph position="11"> 14.a. Q: Are you going to the movies tonight? b. R: \[no\] c. I can't afford to.</Paragraph>
      <Paragraph position="12"> d. I spent $900 on my car last week.</Paragraph>
      <Paragraph position="13"> e. It needed a new transmission.</Paragraph>
      <Paragraph position="14"> f. The car is 15 years old.</Paragraph>
      <Paragraph position="15"> Third, certain syntactic forms for expressing Yes-No questions require more than just yes~no to make clear whether R is confirming or disconfirming that a proposition p holds \[Ste84\]. For example, is (15) shows the use of amplification (i.e. supplying an auxiliary verb) in answer to a negative-polarity, positive-bias  (A question is said to have positive or negative polarity depending on the presence or absence of negation, respectively. Bias describes the questioner's presumed belief in the truth of the questioned proposition.) In answer to (16a), a negative-polarity, negative-bias question, 19 R provides an indirect answer, giving the extent to which the proposition that R has been up18(15) and (16) are Stenstrbm's (105) and (111), respectively.</Paragraph>
      <Paragraph position="16"> 19Although it is expressed as a declarative sentence, Stenstr6m classifies (16a) as a request for confirmation, which we treat as a type of Yes-No question.</Paragraph>
    </Section>
  </Section>
  <Section position="6" start_page="196" end_page="196" type="metho">
    <SectionTitle>
6. Related research
</SectionTitle>
    <Paragraph position="0"> Our work differs from most previous work in cooperative response generation in that, in our model, the information given in an indirect answer conversationally implicates \[Gri75a\] the direct answer. I-Iirschberg \[Hir85\], who provided general rules for identifying when a scalar conversational implicature is licensed, claimed that speakers may give indirect responses to Yes-No questions in order to block potential incorrect scalar implicatures of a simple Yes or No. She implemented a system which determines whether the Yes/No 2deg licenses any unwanted scalar implicatures, and if so, proposes alternative scalar responses which do not. This type of response is similar, in our model, to a speaker's use of Use-contrast motivated by clarifyextent-indicated, as illustrated in (9). (As noted earlier, our coherence rules for the relation cr-contrast as well as our axioms for clarify-extent-indicated make use of the notion of a salient partial ordering which was elucidated by Hirsehberg.) However, Hirschberg's model does not account for quite a variety of types of indirect answers which can be generated using the other operators in our model, nor for other motives for using Use-contrast.</Paragraph>
    <Paragraph position="1"> Rhetorical or coherence relations \[Gri75b, Hal76, MT87\] have been used in several text-generation systems to aid in ordering parts of a text (e.g.\[Hov88\]) as well as in content-planning (e.g. \[McK85, MP93\]).</Paragraph>
    <Paragraph position="2"> The discourse plan operators based on coherence relations in our model (i.e. the operators used as satellites of top-level operators)play a similar role in content-planning of an indirect answer. When coherence relations are used for content-planning, it is necessary to constrain the information which thereby may be selected. McKeown \[McK85\] uses discourse focus constraints. In \[Moo89\], plan selection heuristics are used that maximize the heater's presumed familiarity with the concepts in the text, prefer general-purpose to less general operators, and minimize verbosity. Maybury's \[May92\] system uses &amp;quot;desirable&amp;quot; preconditions, pre-conditions that are not inecessary preconditions, to prioritize alternative operators. In contrast to the above, by the use of stimulus conditions, our model is able to trigger new, opportunistic speaker goals.</Paragraph>
    <Paragraph position="3"> Moore and Pollack \[MP92\] show the need to distinguish the intentional and informational structure of discourse, where the latter is characterized by the 2degHirschberg did not addresss other possible types of direct answers, represented by the Answer-Hedged, Answer-Maybe, and Answer-Maybe-Not operators in our model.</Paragraph>
    <Paragraph position="4"> sort of relations classified as subject-matter relations in RST. Note that in our interpretation component \[GC94\], informational relations (i.e. plausible coherence relations holding between indirect answers and candidate direct answers) are used to infer the speaker's goal to convey a particular answer. Moore and Paris \[MP93\] argue that it is necessary for generation systems to represent not only the speaker's top-level intentional goal, but also the intentional sub-goals that a speaker hoped to achieve by use of a rhetorical relation so that, if a subgoal is not achieved, then an alternative rhetorical means can be tried. In our model, the operators used as satellites of top-level answer discourse plan operators are based on RST's subject-matter relations. The primary goals of these operators are similar to the effect fields of the corresponding RST relation definitions. As Moore and Paris argue, goals based on these relation definitions are rhetorical goals, not intentional goals. However, in our model, it is possible to determine the intentional subgoals by checking which stimulus conditions motivated the speaker to include a particular satellite operator. For example, if explanation-indicated motivates R to include a Use-cause satellite, then one can view R as having the implicit intentional goal of giving an explanation. Thus, our model provides the information necessary to recover from the possible failure of (all or part of) an indirect answer.</Paragraph>
    <Paragraph position="5"> Finally, our use of simulated interpretation during plan pruning has some similarity to previous work. In Horacek's approach to generating concise explanations \[I-Ior91\], a set of propositions representing the full explanation i s pruned by eliminating propositions which can be derived from the remaining ones by contextual rules. Jameson and Wahlster \[JW82\] use an anticipation feedback loop algorithm to generate elliptical utterances.</Paragraph>
  </Section>
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