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<Paper uid="E99-1002">
  <Title>Generating referring expressions with a unification grammar</Title>
  <Section position="4" start_page="0" end_page="10" type="metho">
    <SectionTitle>
2 Representing linguistic context
</SectionTitle>
    <Paragraph position="0"> For any referring expression (e.g. 'a patch') one can define two relevant contextual states: first, the context in which the expression may be used; secondly, the context that results from its use. These will be called the 'initial' and 'final' contexts. In the case of 'a patch', they can be informally defined as follows.</Paragraph>
    <Paragraph position="1"> Initial context: The patch is not in focus, it has not been mentioned before, and no other patch has been mentioned.</Paragraph>
    <Paragraph position="2"> Final context: The patch is in focus, it has been mentioned, and no other patch has been mentioned. null The aim of this section is to model the initial and final contexts formally, considering not just indefinite descriptions but the full range of nominals mentioned earlier (including pronouns, definite descriptions and ordinal descriptions). For this purpose we will discuss an example that includes at least one nominal of each kind.</Paragraph>
    <Paragraph position="3"> To put on a patch:  1. Take a sachet.</Paragraph>
    <Paragraph position="4"> 2. Remove the patch from a second sachet.</Paragraph>
    <Paragraph position="5"> 3. Position the patch and press it firmly.</Paragraph>
    <Paragraph position="6">  The strange second step suggests that the author has made a mistake during knowledge editing, introducing a second sachet instead of re-using the sachet entity introduced in step 1. An important objective of the WYSIWYM feedback text is to expose such errors clearly. Because of this editing mistake, the passage mentions three objects: one patch, and two sachets. The patch is unique, the only object in the discourse satisfying the description 'patch'. The sachets, instead, are distractors -- i.e., distinct objects answering to the same description. null As a first approximation, the contextual state can be formalized by two vectors which will be called the 'focus vector' and the 'mention vector'. Each vector should contain one element for each discourse referent that might be expressed by a nominal referring expression, so that in the example the vectors will be three elements long. The order of elements in the vector is irrelevant provided that it is observed consistently: it will be assumed arbitrarily that it is SA, SB, p, where SA and sB denote the two sachets and p denotes the patch. Note in particular that the order of SA and sB in the vector is independent from their order of introduction in the text.</Paragraph>
    <Paragraph position="7"> The values in the focus vector are boolean: 1 if the referent is in focus, 0 if it is not. We simplify</Paragraph>
  </Section>
  <Section position="5" start_page="10" end_page="11" type="metho">
    <SectionTitle>
3 Incorporating context into the
</SectionTitle>
    <Paragraph position="0"> grammar A requirement on all WYSIWYM systems has been fast response. Every time that the author selects an editing operation on the feedback text, the knowledge base is updated and a new feedback text is generated. Any tangible delay in presenting the updated feedback text is irritating. In pursuit of efficiency, ICONOCLAST employs a top-down generator coupled with a unification grammar. The grammar adheres strictly to Occain's razor: features or rules are admitted only if they contribute to generating the desired texts. ICONOCLAST is implemented in ProFIT (Erbach, 1995), so that feature structures are represented by Prolog terms and can be unified efficiently through Prolog term unification.</Paragraph>
    <Paragraph position="1"> How can linguistic context be fitted into such a scheme? Ideally we would like to incorporate context into the phrase-structure rules, so that for example a rule introducing a pronoun would be applied only if the referent to be expressed had a value of 1 in the focus vector. Unfortunately such a rule could not be formulated in general terms: both its semantic features and its focus and mention vectors would depend on particular properties of the current knowledge base. However, nothing prevents us from constructing 'bespoke' rules, tailored to the current state of the knowledge base, every time that it is updated. At first sight this might seem a ridiculous waste of time -- one would have to envisage beforehand all the ways in which every referent might be expressed -- but in compensation the search phase of generation can proceed much faster, since all calculations relating to linguistic context have already been performed, and there is no danger that they might be duplicated.</Paragraph>
    <Paragraph position="2"> Returning to the example in the previous section, let us work out the bespoke phrase-structure rules that should be added to the grammar so that it can refer to SA, SB and p. At this stage we do not know the exact contexts in which these referents will be introduced; these will depend on text-planning decisions during generation. Nevertheless, some valid generalizations can be made in advance by examining the content to be expressed: * p will be mentioned several times, so we might need pronouns, definite descriptions, and indefinite descriptions. However, since p has no distractors, no rule introducing ordinals will be necessary.</Paragraph>
    <Paragraph position="3"> * SA and SB are mentioned only once each, so definite descriptions and pronouns are unnecessary. However, since they are distractors, indefinite descriptions with ordinals should be provided.</Paragraph>
    <Paragraph position="4"> Here is a phrase-structure rule generating indefinite descriptions for SA (either 'a sachet' or 'a second sachet'). The rule is presented in simplified ProFIT notation, where F!V means that V is the value of feature F; as usual in Prolog, symbols starting with a lower-case letter are constants, while symbols starting with an upper-case letter are variables. Focus and mention vectors are represented by lists, while the phrase-structure constituents are listed under the cset feature. It will be seen that the rule does not rely entirely on unification, because it includes a statement expressing Df as a function of Di, but it will shown later how this blemish can be removed.</Paragraph>
    <Paragraph position="5">  rule (referent ! sA &amp; properties ! \[type :patch\] &amp; syntax !np &amp; initial! (focus! \[0 .... \] &amp; mention! \[O/Di, N/Di, M\]) &amp; final! (focus! \[i, O, O\] &amp; mention! \[Dr/Dr, N/Dr, M\]) cset ! \[properties ! \[type : indef\] &amp; syntax ! det, properties ! \[order: (Dr/Dr), type :patch\] syntax ! nbar\] ) : Df is Di + I.</Paragraph>
    <Paragraph position="6">  The syntactic form of this rule is NP --+ DET + NBAR, where the NBAR can be expanded by NBAR --+ NOUN to yield 'a sachet', and by NBAR --+ ORDINAL + NBAR to yield 'a second sachet'. Which of these rules is applied will depend on the order property, which reproduces the final mention ratio -- a ratio of 1/1 activates the former rule, while any other ratio activates the latter.</Paragraph>
    <Paragraph position="7"> The above statement of the rule simplifies by specifying contextual features only on the parent. In this particular case the omission is harmless: since the sachets have no properties (apart from type), the NBAR of the indefinite description cannot include any expression referring to other objects (e.g. 'a sachet containing a patch'). In general, however, subordinated nominals might modify the context, so the final context of the parent should depend partly on the final context of its last constituent. This requires two things: first, the context must be 'threaded' through the constituents; secondly, the relationship between the final contexts of the parent and the last constituent must be defined.</Paragraph>
    <Paragraph position="8">  by assuming (a) that focus is all-or-none rather than a matter of degree, and (b) that at most one referent can be in focus at any time. Actually the ICONOCLAST system refines the second limitation by grouping the referents according to whether they are competitors for the same pronoun: people compete for 'he/she' (or 'him/her' etc.), and physical objects for 'it'. With this refinement, the relevant constraint is that at most one referent in each group can be in focus at any time. However, in the example, the three referents are all physical objects -- competitors for 'it' -- so this complication can be ignored.</Paragraph>
    <Paragraph position="9"> The behaviour of the focus vector is straightforward. At the beginning of the text no referent has been mentioned, so all focus values are zero:</Paragraph>
    <Paragraph position="11"> Whenever an object is mentioned, it comes into focus and its rivals go out of focus. As a result, the phrase 'the patch' in the final step switches the focus vector to the foUowing:</Paragraph>
    <Paragraph position="13"> With p now in focus, the pronoun 'it' can be employed to refer to p in the final clause.</Paragraph>
    <Paragraph position="14"> The mention vector is more complex. Each value is a ratio N/D, where N is the order of introduction of the referent relative to its distractors, and D is the number of members of the distractor group introduced so far. If the referent has not yet been mentioned, N = 0; if no members of the distractor group have yet been mentioned, D = 0.</Paragraph>
    <Paragraph position="15"> Initially all mention ratios are set to 0/0; at the end of step 1 in the example the state of the mention vector will be as follows (assuming that the first-mentioned sachet is SA):  Consequently, when SB is introduced during the second step, its initial mention ratio is 0/1, meaning that while sB has not yet been mentioned, one of its distractors has got in first: On the basis of this information the generator should produce an indefinite description including the ordinal 'second' (or perhaps the determiner 'another'). By the end of step 2 all three objects have been introduced, so the mention vector reaches its final state:  Note that the two mentions of the patch in step 3 have no effect on the mention vector: its purpose is to record the order of introduction of a referent in relation to its distractors, not the number of times that a referent has been mentioned. When choosing a referring expression it is relevant whether a referent has been mentioned (as signalled by its N value in the mention ratio), but the precise number of mentions is of no significance. null It has been shown that the focus and mention vectors allow us to represent the initial and final contexts of the referring expressions in the example. (Of course we have oversimplified, especially in our treatment of focus.) We now show that by abstracting from the particular contexts in the example, it is possible to describe the initial and final contexts of these referring expressions in all texts expressing the same content. This is done by using variables to represent the values of any contextual features that do not interact with the referring expression under consideration. For instance, the generalized initial and final contexts of 'a patch' are Initial context Final context p 'a patch' SA SB p SA SB p</Paragraph>
  </Section>
  <Section position="6" start_page="11" end_page="12" type="metho">
    <SectionTitle>
FOCUS FA FB 0 0 0 1
MENTION MA Ms 0/0 MA MB 1/1
</SectionTitle>
    <Paragraph position="0"> where FA, MA, etc. are variables. Among other things this rule implies that 'a patch' may be used whatever the current focus values for SA and SB, but that after &amp;quot;a patch' these objects must be out of focus. Here are the corresponding rules for the other referring expressions in the example.</Paragraph>
    <Paragraph position="1">  Note that each rule is specific to a referent. For instance, the rule given for 'a sachet' is specific to SA; a slightly different rule would be needed to describe the contexts in which 'a sachet' can be employed to refer to SB.</Paragraph>
    <Paragraph position="2">  Proceedings of EACL '99 The procedure for threading contextual features is straightforward. Suppose the rule has the form u0 -+ ui + u2... + uN, and that the initial and final contexts of any unit u are I(u) and F(u). In all cases, the initial context of the parent should be unified with the initial context of the first daughter, so that I(uo) = I(ui). The relationship between I(ui) and F(ut) will depend upon the rule that expands the first daughter, but the final context of any daughter should always be unified with the initial context of the next daughter, so that for example F(ut) = I(u2). Moreover, for any rule that does not generate a referring expression, the final context of the last daughter can be unified with that of the parent, so that F(ug) = f(uo).</Paragraph>
    <Paragraph position="3"> For referring expressions, instead, F(uo) usually differs from F(ug), because the end of a referring expression is the point where the linguistic context may be changed.</Paragraph>
    <Paragraph position="4"> Thus to take account of subordinated referring expressions, a rule must specify the relationship between three contexts: I(uo), F(uiv), and F(uo). A rule capable of expressing SA by 'a sachet contalning a patch' should represent these contexts as follows:</Paragraph>
  </Section>
  <Section position="7" start_page="12" end_page="12" type="metho">
    <SectionTitle>
MENTION Dr~Dr N/Df M
</SectionTitle>
    <Paragraph position="0"> where D/= Di + 1.</Paragraph>
    <Paragraph position="1"> Finally we return, as promised, to the problem of updating mention ratios by unification, without resorting to statements like Df is Di + 1. This can be done by replacing numbers with lists of the appropriate length, so that for example the ratio 0/2 is represented by the term \[\] / \[_, _\] With this convention, the relationship between the mention ratios of F(UN) and F(uo) can be stated without an accompanying numerical constraint: null</Paragraph>
    <Paragraph position="3"/>
  </Section>
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