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<Paper uid="W99-0105">
  <Title>O @ O O O 0 O O @ O O @ O 0 O O @ Positing and resolving bridging anaphora in deverbal NPs</Title>
  <Section position="2" start_page="0" end_page="39" type="metho">
    <SectionTitle>
I Introduction
</SectionTitle>
    <Paragraph position="0"> This paper explores the idea that the operations of positing and resolving bridging anaphom in NPs with deverbal heads can be successfully guided by considering as possible implicit relations those suggested by the argument structures of the correspondink verbs.</Paragraph>
    <Paragraph position="1"> Bridging is a referential phenomenon occurring when the referent of ~t linguistic expression can be determined only by recovering a meaningful implicit relation with an already mentioned entity or event.</Paragraph>
    <Paragraph position="2"> For example, in the following Italian sentences, the correct interpretation for richieste requires the identification of the implidt relation between the set of requests for underwriting and the bond to be underwritten. null ex. Un nuovo prestito obbli~izionor~o al tasso del 7% e della durata di ire anni verrk emesso domani dal Banco Ambroveneto, .... Le r/chiate verranno accettste dal 2 al 14 ottobre.</Paragraph>
    <Paragraph position="3"> \[A new bond Ioan...will be ~ued tomorrow by Banco Ambroveneto ..... The requests will he accepted from the 2nd to the 14th of October.\] The literature describes two basic approaches to deal with bridging in CL: the first consists in working mostly at the semantic level interpreting bridging as a kind of implicature the reader draws to support the coherence of discourse (Asher and Lascarides, 1998, pp.l-2). The s~cond, exemplified by (BOS, Buitelaar, and Mineur, 1995), relies primarily on augmenting the lexicon annotating for each noun its possible meaningful relations. We believe that, at least for the particular kind of bridging we are concerned with, the second approach is most promising given that, in some cases, a limited amount of lexicon annotation can greatly enhance resolution efficiency and efficacy: by using algorithms and rules relying on linguistic information we can significantly constrain inference on cognitive processing. Our contribution goes along the lines proposed in (Grc~ and Sidner, 1998).</Paragraph>
    <Paragraph position="4"> We are interested in deverbal nouns (e.g. ~buildinK&amp;quot; derived from %o build&amp;quot;, and &amp;quot;request&amp;quot; derived from &amp;quot;to request &amp;quot;) and we endorse the claim that bridging occurring in NPs with a deverbal noun head should be resolved by considering as possible implicit relations those suggested by the argument structure of the corresponding verb, cf. inter alia (Badia and Saurl, 1998) and (Rozwadowslm, 1988) for investigations on thematic restrictions on derived nominals. We think that the way the verb argument structure maps onto the noun argument positions depends on whether the deverbal nouns denote an event,.or a resulting state or part/c/pant in the corresponding eventuality. The ides of exploiting the event/result reading distinction, combined with other aspectual information, was primed by the linguistic data collected during corpos analysis and proved to be quite promising. We then take it one step forward and work out which is the noun argument whose (covert or overt) presence is needed for a proper understanding of the text. This idea is captured by the notion of/ovourite.</Paragraph>
    <Paragraph position="5"> Our work fits in the context of automatic information extraction. In connection with the FACILE Europeanproject (LE 2440), we developed a module for texture resolution I (TRM) to be integrated in a system of information extraction from Italian financial news. The TRM tracks entities as they IThe te.~ure of a text is &amp;quot;related to the listener's perceptio, of coherence&amp;quot; and is &amp;quot;manifested by certain kinds of semantic relations \[called cohesive ties\] between i~ individual message&amp;quot; (Hallidsy and Hssan, 1985). Examples of cohesive ties are: coreference, bridging and coclassification.</Paragraph>
    <Paragraph position="6">  are introduced in the discourse and uses a set of declarative rules to guess Which cohesive ties hold for a certain referring expression. An exploratory corpus analysis (on 13 Italian financial news for a total of 1467 words) showed that bridging is a frequent anaphoric phenomenon (43,3% of the total cohesive links). Moreover, NPs with a deverbal noun as head amounted to 19.6% of the NPs in the corpus and 21% of the bridgings came from deverbals. Building on observations made on our corpus, we first formulated hypotheses for bridging resolution in linguistic terms. Then we went back to our corpus to verify their consistence with respect to the data. Finally, we turned them into heuristics that help recoguise the thematic role played by modifiers in nominalisations and that suggest where missing arguments should be recovered from the discourse context.</Paragraph>
  </Section>
  <Section position="3" start_page="39" end_page="42" type="metho">
    <SectionTitle>
2 Properties of deverbal
nominalisations
</SectionTitle>
    <Paragraph position="0"> There is an assumed parallelism in argument structure between deverbal nouns and the verbs to which they are related, but its strength is said to vary.</Paragraph>
    <Paragraph position="1"> (Grimshaw, 1990) distinguishes between event nominals, that express an event or a process whose existence is entailed, and restdt nominals, that name/denote the output of the event or an entity related to it but do not entail the existence of the corresponding event. Event nominals obligatorily have an argument structure derived from the associated verb and assign specific thematic roles. For instance in: ex. La costruzione della C/asa richiese tre anni \[The construction of the house took three . yell /a casa is assigned the role of patient in the therustic grid of the corresponding verb costruirC/.</Paragraph>
    <Paragraph position="2"> Result nominals, by contrast, take a kind of semantic participants that are more loosely associated with the argument structure and the corresponding thematic roles. For instance, the di NP phrase in ex. la costruzione di Daniele &amp; la pih bells \[ the construction of Daniele is the nicest\] can be interpreted as referring to the agent, i.e. the constructor, but also to the mere possessor of the building. Encoding in the lexicon the event/result reading distinction for a nominalisation is straight-forward when using a conceptual dictionary like WordNet (Fellbanm, 1998): it's enough to search up in the hierarchy. For example, the noun building belongs to different synsets (i.e. it has different senses): one has ent/ty among its hypernyms (and thus it expresses a resu/t) and the other two have act as hypernym (and thus they express an event).</Paragraph>
    <Paragraph position="3"> Thus this noun has both event and restdt, readings.</Paragraph>
    <Paragraph position="4"> &amp;quot; 40 Tile difference in the way argu~fi'ents are realised iu tile two readings is relevant for our purpose. In Lhe La.~k of Lexture resolution, we have to find out precisely how to interpret PP modifiers z, possessires and other potentially referential expressions surrounding a nominal, and when to posit a bridging anaphora and to which entity.</Paragraph>
    <Paragraph position="5"> Nominalised expressions in English and Italian do not share always the same syntactic forms. English nominals have several argument positions that can map into the basic verb arguments (subject, direct and indirect objects), cf.(Macleod et al., 1997) for a computational treatment. They are (i) possessives (e.g. &amp;quot;His announcements&amp;quot;, &amp;quot;Daniel's appointment*); (ii) pre-nominal noun modifier (e.g.</Paragraph>
    <Paragraph position="6"> &amp;quot;The State Department announcement&amp;quot;); (iii) post-nominal prepositional phrase (e.g. q'he announcemeat of the White House&amp;quot;). In Italian, the grammar of nominalisation phrases is somewhat simpler. Arguments can be expressed either by a .possessive 3 or relative pronouns, classified as prenominal moditiers (e.g. ~il suo annuncio&amp;quot; \[his announcement\], ~il cui annuncio&amp;quot; \[whose announcement\]), or PPs, here called postnominal modifiers (e.g. &amp;quot;l'annuncio del ' presidente&amp;quot; \[the announcement of the president\]), cf. (Renzi, 1991) for a description.</Paragraph>
    <Paragraph position="7"> Only PPs of the form di NP (lit. o\[ PP in English) raise interesting questions from our point of view, since PPs other than di NP can be interpreted directly by looking at the argument structure of the related verb, broadly speaking. The major difficulty comes from roles assigned to verb argument positions which are not 'identified' by specific prepositions, because these roles can be mapped into the same position in the corresponding nominals. Indeed, nominals derived from transitive verbs potentially bring in ambiguities, since the thematic roles of agent and patient can both be assigned to possemives and PPs of the form di NP (e.g., agent: &amp;quot;In telefunata di Maria&amp;quot; \[the telephone call by Maria\] vs. patient: 'Temissione di nuove asioni&amp;quot; \[the new share issue\]). Besides, note that a PP of the form di NP can discharge also other roles, for instance an oblique such as material (e.g. &amp;quot;In costruzione di mattoni&amp;quot; \[the construction made of bricks\]). FUrthermore, it can be used to exp~ roles in relations that are not necessarily part of an argument structure, such as possession.</Paragraph>
    <Paragraph position="8"> The literature offers discussions on patterns for coding nominalisations and their arguments (Meyers et al., 1998), (Badia and Sanri, 1998). It is noted that whether or not a position is filled may affect the interpretation of other positions. It is also dis~Here modifiers is a cover term for complements, adjuncts and so on.</Paragraph>
    <Paragraph position="9"> aNote that morphological agreement goes with the nominal. Semantic agreement (with the possessor) is not .allowed.</Paragraph>
    <Paragraph position="11"> cussed the status of 'optional' arguments to nouns.</Paragraph>
    <Paragraph position="12"> In some sense, these papers try to give 'maximal; descriptions of nominalisations. Our concern is different. We want t~ identify the texture of a text.</Paragraph>
    <Paragraph position="13"> Therefore, we want to know when a bridging can be safely posited and how to resolve it. To this aim, 'minimal' descriptions are best suited because they trigger the hypothesis that there is a bridging in a reduced number of cases, with a higher degree of certainty and provide precise information for its resolution. We capitalise on the observation that in the interpretation Of a nominalisation there are preferences in the assignment of a certain thematic role in a certain context, and that one of the basic complements appears to be singled out. This type of preference is different from the distinction between obligatory and optional complements. We describe this situation via the notion of favourite argument which has semantic and syntactic facets: * From a semantic point of view, the notion of favourite captures the following idea. For each nominal, there is a thematic role (the favourite argument) that must be assigned for the referent to be correctly situated wi'thin the discourse context. For example, for the Italian nominal costruzione (construction) derived from the verb costruire (to construct) the favourite is the thematic role of patient. In the sentence: ex. La costruzione della cas~ r/chiese tre aani.</Paragraph>
    <Paragraph position="14"> \[The construction of the house took three years.\] the- patient (i.e. the house) is essential for the interpretation of the sentence. Were it miss- lag, as in ~La v0s.truzione dchiese tre anni&amp;quot;, we * would be forced to search in the previous discourse for a coreference or a bridging link on the construction.</Paragraph>
    <Paragraph position="15"> We hypothesize that the favourite role is unique, even if not nw.essadly the same for all nominab. It can be overfly assigned, by mapping it into the unique argumental position reaIised, provided it is of a compatible form 4, or into one of the arguments if there are more. Otherwise, one must assume that there is an understood relation of coreference or bridging. Given our hypothesis that the favourite role is unique and provides the minimal and sufficient information to recover the cohesive texture of discourse, no bridging is posited on arguments other than the favoudte.</Paragraph>
    <Paragraph position="16"> Evidence in support of a unique favourite comes from the consistent preference recorded in naturally occurring instances, and from the use of 4SelectJonaJ restriction or other semantic checks are used to test compatibility. An ex~npJe of non-compatible form is to coltsmzione d; ie~ \[the construction of yesterdsy\].  recovering strategies in case 0fpotential mismatches between the favourite's qualification and world knowledge. For instance, in &amp;quot;In costruzione di Daniele richiese tre anni&amp;quot; \[the construction of Daniele took three years\], the predicate forces a reading as event of the deverbal. The favourite is the role of patient, whereas world knowledge would prompt the role of agent for di Daniele. As a matter of fact, the primary interpretation of this sentence is something like 'the construction of the statue portraying Daniele ...', which accommodates the role of patient.</Paragraph>
    <Paragraph position="17"> From a syntactic point of view, the notion of favourite expresses the fact that there must be an explicit or implicit modifier to which the favourite thematic role is assigned. Both possessires and PPs of the form di NP are compatible forms for the semantic favourite, but in case of competition, the PP is preferred. The favourite position is the one closest to the noun.</Paragraph>
    <Paragraph position="18"> Our notion of favourite argument differs from the notion of core thematic role in (Barker, 1995) in at least three respects. First, the favourite does not identify a role discharged by the deverbal itself, as it i~ the case of the core role. Instead, it expresses a condition applying to a (overt or covert) modifier ofthe deverbal. Second, it is not used to encode the distinction between event and result readings of the deverbal. Indeed, such a distinction is presupposed for the favourite to be computed. Third, it identifies a participant in the relation expressed by the cleverhal. Moreover, the favourite does not identify the relation itself via the instantiation of the event variable, for the cases of event reading, nor the role that is discharged by the deverbal, for theresult reading, as in (Barker, 1995). Note that this last point does not preclude the possibility of having event positions in the argument list of the predicate representation of the corresponding verb.</Paragraph>
    <Section position="1" start_page="41" end_page="42" type="sub_section">
      <SectionTitle>
2.1 Heuristics for thematic roles
assignment
</SectionTitle>
      <Paragraph position="0"> Starting from observations made on our corpus of Italian financial news, we define heuristics for the identification of the semantic favourite using aspectual information. Beside the event/result dichotomy, we further distinguish results as originating from statire (to/mow) or eventive (to construct) predicates.</Paragraph>
      <Paragraph position="1"> We also distinguish eventives where the existence of a participant is affected by the existence/happening of the event described by the predicate (to construct a home) from eventivas where it is not (to capture a lion). Let's call the former an E(xistence) A(ffecting) type of predicate. Our hypothesis is that this distinction is more relevant than other aspectual subsp~.cifications because&amp;quot; we are concerned with referential issues.</Paragraph>
      <Paragraph position="2"> We worked out the following heuristics for the interpretation of modifiers: * Whenever there are several PP modifiers, the favourite thematic role must be assigned to the modifier closest to the nominal which has a compatible form. World knowledge tells us that collegio and convento are collective entities that can be agent in an event'of construction or buildings and hence be patient. However, the following example allows only the interpretation of patient for collegio.</Paragraph>
      <Paragraph position="3"> ex. La coetruzione del collegio del convento \[The construction of the boarding school of the convent\] As for convento, it can be the agent or the posse~or. null * In the absence of a modifier that can act as the favourite, there must be coreference or a bridging anaphora on the favourite.</Paragraph>
      <Paragraph position="4"> * For a nominalisation which is a restdt:.</Paragraph>
      <Paragraph position="5"> If it is an EA predicate, the nominal denotes the entity affected by the event, e.g. the patient, and the corresponding theta role cannot be discharged overtly. The search space for the interpretation of the modifiers is pruned accordingiy. The favourite is the agent but, because a result is an object and not an event, other relations such as possession cannot be ruled out: ex. La costruzione di Maria venne giudicata come la pitt solida.</Paragraph>
      <Paragraph position="6"> \[The construction of Maria was considered as the firmest.\] If it is not an EA predicate, then patient is the first role to be discharged in the case of an eventive predicate.</Paragraph>
      <Paragraph position="7"> ex. La cattura di Made ci sorprese.</Paragraph>
      <Paragraph position="8"> \[the capture of Marie surprised us.\] In the cue of a stative predicate, a modifier is more likely to diKharge the role of agent/~r.</Paragraph>
      <Paragraph position="9"> ex. La conoecenza di Daniele ~ sconflnsta.</Paragraph>
      <Paragraph position="10"> \[Daniele's knowledge is unlimited.\] * For a- nominalisation which is an event the role assigned to the direct object of the corresponding predicate must be discharged by the favourite s and it is discharged by a PP of form di NP.</Paragraph>
      <Paragraph position="11"> SFor the sake of brevity, let's assume that it is always the patient.  ex. La Costruzione della casa fu lunga e laboriosa.</Paragraph>
      <Paragraph position="12"> \[The construction of the house was long and tiresome.\] If there are more than one PP modifier, the PP discharging the patient role occurs closest to the nominal.</Paragraph>
      <Paragraph position="13"> in cases where a deverbal nominalisation may have in principle both result and event readings, as in the case of costruzione, some additional aspectual or semantic information can help to discriminate the reading occurring in the discourse context under consideration: * Modifiers may have discriminating power: If there is a modifier that can be identified as the patient (we can use standard selectionai restrictions such as animate...), then the reading as result is ruled out, as in: ex. La costruzione della casa ...</Paragraph>
      <Paragraph position="14"> \[The construction of the house ... \] On the contrary, if there is a single modifier, which is a PP of form di NP, that can be identified unambiguously as the agent, then the nominal is a result, as in: ex. La costruzione del bambino...</Paragraph>
      <Paragraph position="15"> \[The construction of the child... \] 0 ff information on the role of the modifiers helps in interpreting/identifying the type of nominal and vice versa, we fall in a bit of a circular procem. However, sometimes the discriminating power of predicates can help in disambiguating the word sense. If the predication is about the temporal dimension, then the reading as result can be ruled out, as in the building was slow. If the prech'cation is about the physical/spatial dimension, then the nominal is a result, as in the building is high.</Paragraph>
    </Section>
    <Section position="2" start_page="42" end_page="42" type="sub_section">
      <SectionTitle>
2.2 Corpus based verification of the
</SectionTitle>
      <Paragraph position="0"> heuristics The ~ exploratory corpus analysis we conducted on Italian financial news confirmed the validity of the linguistic hypothesis given above. Texts were annotated systematically with an SGML tagging: for each deverbal nominalisation the reading (whether event or result) and aspectual class (whether state, activity, accomplishment or achievement) are marked, and for each corresponding moditier, its linguistic form and its thematic and syntactic role with respect to the originating verb are specified. See figure 1 for a sample of annotated text. Contrary to what we expected, the corpus analysis revealed that the aspectual class does not condeg tribute significantly to the positing and resolving of  &lt;news id = &amp;quot;10&amp;quot; &gt; La Banca Agricola Mantovana ha deliberato di dar corso ad &lt;np id = 60 &gt; una &lt;head type = &amp;quot;event&amp;quot; aspect .= &amp;quot;accomplishment&amp;quot; &gt; emissione &lt;/head&gt; &lt;pp prep -- &amp;quot;di&amp;quot; theta = &amp;quot;patient&amp;quot; syntax = &amp;quot;directobject&amp;quot; &gt; di obbligazioni non convertibili &lt;/pp&gt; &lt;pp prep =&amp;quot;di&amp;quot; theta = &amp;quot;measure&amp;quot; syntax -&amp;quot;adjunct&amp;quot; &gt; dell' importo complessivo di 100 miiiardi di life &lt;/pp&gt; &lt;/np&gt; ...... L' &lt;head type = &amp;quot;event&amp;quot; aspect -- &amp;quot;actlvlty&amp;quot; &gt; offerta &lt;/head&gt;  bridgings, given that the event/result distinction is a sufficient predicting factor. Table 1, summarizes our corpus-based findings.</Paragraph>
      <Paragraph position="1"> In the table, under the column &amp;quot;other&amp;quot; we have grouped occurrences of deverbal nominalisations whose analysis falls outside the scope of this research. They are coreferential anaphora, specific refo erences, generic references, nominals with di PP expressing the only obligatory thematic role, nominals with bridging on the only obligatory thematic role.</Paragraph>
    </Section>
  </Section>
  <Section position="4" start_page="42" end_page="43" type="metho">
    <SectionTitle>
3 Computational use of the
</SectionTitle>
    <Paragraph position="0"> heuristics for positing and resolving bridging The heuristics described above can be profitably exploited in a system for automatic information extraction to improve the efficacy of the texture resolution module.</Paragraph>
    <Paragraph position="1"> The Texture Resolution Module (TRM) we developed in connection with the FACILE project tries to identify the reference function that each NP plays in a text (anaphora, generic reference, specific reference, iota (unique) reference, predicative function) and tries to guess possible cohesive ties (Not and Zancanaro, 1998). The key idea underlying the TRM design is to coadder the process of texture resolution as a three step process: I. for each referring expression, determine the reference function which may apply according to the linguistic form of the NP (for example, pronouns are never considered as potential iotas); 2. for each guess of anaphoric reference function. look for potential antecedents (coreference is tried first; if this search fails or is not satisfactory, try other cohesive ties, like bridging or coclassification): (2.a) select the search space where to look for antecedents; (2.b) collect all the entities in the search space which can be taken as referent for the considered expression. Different tests are applied, for example checking semantic compatibility and eliminating redundant solutions; null 3. for each sentence, (3.a) test the compatibility of the guesses made separately on each referring expression wrt intrasentential constraints (for example, C-command and contraindexing); (3.b) order the alternative guesses according to intersentential preferences (like centering).</Paragraph>
    <Paragraph position="2"> The heuristics presented in SS2.1 can be used to refine step 2 in the algorithm: 2. for each guess of anaphoric reference function: * if the NP is a deverbal nominalisation then: -look for potential antecedents for a coreference.</Paragraph>
    <Paragraph position="3"> - If a hypothesis of coreference Could not be verified (or is too weak for the TRM to be completely satisfied with it), then look for possible bridgings. As a first step, look in the conceptual dictionary and: A ) If the nominaiisation has only one possible reading (either result or event) then apply the appropriate heuristics for interpretingmodifiers: - if a compatible modifier is found that discharges the favourite, then no other bridging relations are looked for; * if there are no compatible modiRers, so that the favourite role cannot be discharged, a bridging is guessed and the potential antecedent is looked for in the previous discourse as in steps (2.a) and B ) If the deverbal nominalisation can have both result and process readings, then: * verify whether semantic checks on the modifiers or other aspectual informatlon help to discriminate between the two readings, using the heuristics. If a discrimination succeeds, then proceed to step \[A\].</Paragraph>
    <Paragraph position="4"> * otherwise, try to guess possible bridgings for both result and process readings.</Paragraph>
    <Paragraph position="5"> eF'or the sake o/'simpliclty, the discussion t'ocusses only on anaphoric links. Howe~fer, these observations can be general.i~cl to cataphora too.</Paragraph>
    <Paragraph position="6">  agent in favourite position as di PP patient in favourite bridging position as di PP on agent  select the search space where to look for antecedents; collect all the entities in the search space which can be taken as a referent for the considered expression. Different tests are applied, for example, to check for the semantic compatibility and to eliminate redundant solutions;</Paragraph>
  </Section>
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