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<Paper uid="W94-0316">
  <Title>Building Another Bridge over the Generation Gap</Title>
  <Section position="2" start_page="0" end_page="139" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"/>
    <Section position="1" start_page="0" end_page="137" type="sub_section">
      <SectionTitle>
1.1 Identifying the Generation Gap
</SectionTitle>
      <Paragraph position="0"> In text generation, content selection and discourse organization (i.e. the text planning tasks) have often been dealt with independently from the linguistic realization of the tezt plan- the information selected and structured by the text planning process (eft, e.g., \[McKeown and Swartout, 1987\]). However, a text planning process which does not take into account linguistic resources that are available to express a particular meaning suffers from two major shortcomings: (i) it is not sensitive to variant discourse organizations at the sentence level; and (ii) it cannot guarantee that its text plan is always verbalizable by the linguistic module. With other words, there is a discrepancy (a &amp;quot;generation gap&amp;quot; in Meteer's terms) between a text plan that is not tailored to linguistic resources and the input as required by the linguistic realization module. For extensive examples that illustrate this, see, especially Meteer's work \[Mercer, 1991\] and \[Mercer, 1992\], but also, e.g., \[Rubinoff, 1992, Vander Linden et al., 1992\]. In our work, we have found that especially lexical phenomena (such as lexical cooccurrence and lexical semantics) play an important role in discourse organization at the level of sentence planning, which is still one of the unsolved problems in text planning, cf. \[Hovy, 1991\]. Consider the following example: (1) a. Opa schofl auff den Einbrecher, der nun tot ist lit. 'Grandpa shot at the burglax who is now dead'.</Paragraph>
      <Paragraph position="1"> The interpretation of (la) does not necessarily suggest that burglar's death is in consequence of grandpa's shooting. To communicate this relation, (lb) is more  appropriate: (1) b. Opa sehofl au\] den Einbrecher und  tJtete ihn/ verwundete ihn tJdlich lit. 'Grandpa shot at the burglar and killed him/wounded him deadly'.</Paragraph>
      <Paragraph position="2"> However, despite an analogous syntactic construction, (2) suggests the interpretation that grandpa's being well is in consequence of doctor's curing: (2) Der Arzt heilte Opa, der nun ganz gesund ist \].it. 'The doctor cured grandpa, who is now completely well'. That is, the consideration of lexical phenomena helps to resolve the discrepancy between discourse structure relations (as, e.g., given by Rhetorical Structure Theory (RST) \[Mann and Thompson, 1987\]) and their linguistic realization. Therefore, a text generator has to provide an organization of lexical resources which makes explicit the rhetorical potential of lexical phenomena and which allows for the purposeful choice of these phenomena.</Paragraph>
      <Paragraph position="3"> In principle, three ways seem to be feasible for how to proceed: (i) to broaden the task of text planning by lexical choice (eft, e.g., \[Mercer, 1992\]), (ii) to realize an interaction link between the linguistic realization module and text planning (eft, e.g., \[Rubinoff, 1992\]), or Off) to broaden the task of linguistic realization in order to deal with a final, lexically guided determination of a discourse organization that has been predetermined by text planning.</Paragraph>
      <Paragraph position="4">  meaning 'Grandfather shoots at a burglar; in consequence, the burglar is dead.' &amp;quot; spelling: ~schiellen ~ &amp;quot;~ 1 / \[sp~tUag: &amp;quot;Op-&amp;quot;l \[actor: I~LEX-OPAlperson: + \] / Lsin~'.da~: + J ,,  |\[spelling: &amp;quot;Zlnbrecher \] \[ actee: LEX-EINBRECHER\[person: + | / L ndegun: + J Ltheme: la\] * &amp;quot;~'0 .... -~pe\].ling: ~verwunden ~\] ,. ..... Lv~b: / \] ', m ......... A ..... \[spe\].ling: &amp;quot;t~illch'\] ................ \[qutdJty: / \] \[spellihg: &amp;quot;Opa '~\]  actor: LEX-OPA.I person: / \[ Lsi&amp;quot;v ai~'- + ,i \[spelling: &amp;quot;Einbrecher&amp;quot;\] actee: LEX-EINBRECHERI person: /  |Lsms~.l~: + J .theme: \[\] .~ In our work, we follow (iii). This is, in order to keep s separate level of discourse organization that is realized independently from linguistic resources and to make use of the potential of linguistic resources to guide the discourse organization at a more detailed level than conventional text planning is able to do. In contrast to, e.g., \[Fawcett et al., 1992\] and \[Elhadad and Robin, 1_992\], who deal with lexically biased sentence organization within the grammar, we take a radically lexicalist position (similar to that of \[McDonald, 1991\]) in that we assume that lexicalization has to take place before grammatical realization; and that it is the words chosen, which dictate the possible syntactic realizations of a content to be communicated. More precisely, we propose a two level discourse organization where the first level is provided by an RST style text planner and the second level by a separate lexical choice module. Then, the discourse organization of a text is done in two steps: in the first step, the text planner predetermines the discourse structure relations and selects the content; in the second step, the lexical choice module provides, in accordance with linguistic constraints, a finer specification and the realization of these relations and tailors their content to linguistic realization.</Paragraph>
    </Section>
    <Section position="2" start_page="137" end_page="138" type="sub_section">
      <SectionTitle>
1.2 The Framework
</SectionTitle>
      <Paragraph position="0"> The computational framework in which our lexical choice model has partially been implemented is the systemic text generator KOMET \[Bateman et al., 1991\].</Paragraph>
      <Paragraph position="1"> One source of constraints for the first level text organization comes in KOMET from an RST-based planner, z The output of this planner is a collection of case frames with RST relations holding between them as shown in  auf den Einbrecher und verwundete ihn tSdlich lit 'Grandpa shot at the burglax and wounded him deadly' module has to construct an input to the grammar (in KOMET m a systemic grammar of German \[Teich, 1992\]). This input contains the lexicalization of the text plan content and constraints for its syntactic realization. Figure 2 shows such an input (called Partial Grammatical Structure (P6S)) 2 encoded in terms of Typed Feature Structures (TFSs) \[Emele, 1989\]. This input has been derived from the text plan in Figure 1.s In what follows, we present an ongoing attempt to define a general model of lexical resources which would provide the missing link between the global discourse organization as given by discourse structure relations in the text plan and its linguistic realization. In particular, we show how Mel'~uk's Lezical Functions (LFS) (cf., e.g., \[Mel'~uk and Polgu~re, 1987\]) can be used to compile such a general model. After the introduction of the general principles that underly our organization of lexical resources, we focus on LF sequences. We demonstrate that they may well function as lexically biased discourse structure relations at the intrasentential level; and, thus, be interpreted as a finer specification of global discourse structure relations.</Paragraph>
      <Paragraph position="2"> 2A oos corresponds, roughly speaking, to the Partial Surface Functional Description (PSFD) specification in the COMEr system \[MeKeown et aL, 1991\].</Paragraph>
      <Paragraph position="3"> SThe current implementations of the lexical choice module and the grammar still require an additional mapping between the pGs and the input ax taken by the grammar. Since this mapping is, however, purely due to implementational details, it is of no interest here.</Paragraph>
    </Section>
    <Section position="3" start_page="138" end_page="138" type="sub_section">
      <SectionTitle>
sources Organization
</SectionTitle>
      <Paragraph position="0"> While designing a lexical choice model, it is important to note that (a) lexical phenomena and, therefore, also lexical choices that are available to communicate a specific meaning are dependent on this meaning; and (b) lexical phenomena are involved at different stages of the utterance construction process. Thus, the process of SHOOTING has another lexiealization potential than that of CURINC; and the choice of fierce in The terrorists put up fierce resistance is done at another stage than the choice of put up. This calls for: * a (fairly) excaustive coverage of the lexical potential of each semantic entity (such as process, event, etc.); * clustering of lexical resources locally to semantic units of a certain size; * a multilayered organization of lexical resources (and therefore also a multilayered lexical choice process).</Paragraph>
      <Paragraph position="1"> In our model, the resources are provided mainly by Lezical Functions as introduced in the Meaning Text Theory (MTT). They are clustered locally to situations and organized into four layers.</Paragraph>
    </Section>
    <Section position="4" start_page="138" end_page="138" type="sub_section">
      <SectionTitle>
2.1 Using Lexical Functions as the
Source of Lexical Resources
</SectionTitle>
      <Paragraph position="0"> Most excaustively, hxical phenomena have been described in MTT by PSexical Functions.</Paragraph>
      <Paragraph position="1"> Roughly speaking, an LF f is a standard semanticolexical relation which holds between a lexeme L, (the keyword of f) and a set of lexemes f(L) (the value of If). Consider the following examples:</Paragraph>
      <Paragraph position="3"> Mel'~uk distinguishes about sixty simple LFS of the above kind. Simple LFs can further be composed with each other; the meaning of such complex LFs is, as a rule, a composition of the meanings of the participating LFS. Thus, AntiMagn means 'slightly' (e.g., An-</Paragraph>
      <Paragraph position="5"> (shooting)). If several (simple or complex) LFs compose a phrase or a clause (as, e.g., AntiMagn and So compose minor injury), we separate these LFs by a 'o' sign.</Paragraph>
      <Paragraph position="6"> In addition to complex LFs, we introduce LF sequences -- pairs of LFS (denoted LF1 A LFz), whose values cooccur at the syntagmatic axis. For</Paragraph>
      <Paragraph position="8"> shot. LF sequences are directed, i.e. LFx A LF2 LF2 A LFx. Consider, e.g., Prepar A V0, which is an instantiation of a TEMPORAL SEQUENCE relation with the first LF expression preceding the second on the temporal axis, and V0 A Prepar as in He shot after charging the gun, where the first LF expression succeeds the second. Moreover, the existence of LFx A LFz for an entity in a language does not mean that LF2 A LFz is also available (consider, e.g., the following instantiation of A0 A Magn o So: She is beautiful -- a real beauty and Magn o So A A0: *She is a real beauty -- beautiful. Therefore, in LF sequences, one argument is the 'hub' m the point of departure (or the expanded LF) and the other argument is the 'hub expander'.</Paragraph>
      <Paragraph position="9"> LFs and LF sequences (henceforth, both are referred to as 'LFS') have well-defined syntactic realizations.</Paragraph>
      <Paragraph position="10"> For example, Operz is defined as taking the ACTOR as grammatical'subject and the keyword of the situation as direct object. The LF sequence Operl A Magn o So -- when applied to SHOOTING m can be realized as a paratactic complex clause (of. Grandpa took a shot at the burglar; it was a good shot.~ or as a simple clause (eft Grandpa took a good shot at the burglar), but not, e.g., as a hypotactic complex clause (eft *Grandpa took a shot at the burglar, which was a good shot/one). And so on.</Paragraph>
    </Section>
    <Section position="5" start_page="138" end_page="139" type="sub_section">
      <SectionTitle>
2.2 Compiling Lexical Resources Lo-
</SectionTitle>
      <Paragraph position="0"> cally to Situations In accordance with \[Apresjan, 1974\], a situation is comprehensively defined by: 1. a predicate; 2. the constituents (i.e. potential participants and circumstantials) of this predicate; 3. the relations between different constituents as well as between constituents and the predicate (these relations are usually given in terms of actions, states, etc.); 4. the features of the predicate, constituents, and the above actions and relations. null The lexical resources of a situation (the Lezicaliza- null given by all possible wordings of the entities this situation is defined by. For example, SHOOTING is a situation with the predicate named shooting, shot, etc.; with the participants ACTOR (named gunman, marksman, etc.) and ACTEE (named target, victim, ...); with the obligatory circumstantials INSTRUMENT (r/fie, gun, etc.) and MEANS (bullet), and optional circumstantials LOCATION, TIME, etc., which do not have a situation-specific lexicalization. Among the ACTOR'S actions are shoot, fire, kill, wound, etc. The lexicalizations of the relations between ACTOR and the predicate include, e.g., the performance of the situation (fire (a shot), take (a shot), etc.); those between ACTOR and ACTEE -- aim (at), hit, etc. The features of ACTOR include skilled, good, lousy, .... And so on.</Paragraph>
      <Paragraph position="1"> The various LFs -- with the label of a situation as keyword -- supply us with all lexical expressions that communicate the entities this situation is defined by; see the list above for examples with respect to</Paragraph>
    </Section>
  </Section>
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