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<Paper uid="P98-2202">
  <Title>DiMLex: A lexicon of discourse markers for text generation and understanding</Title>
  <Section position="4" start_page="1238" end_page="1239" type="metho">
    <SectionTitle>
2 Building a Discourse Marker
</SectionTitle>
    <Paragraph position="0"/>
    <Section position="1" start_page="1238" end_page="1238" type="sub_section">
      <SectionTitle>
Lexicon
2.1 The idea
</SectionTitle>
      <Paragraph position="0"> The traditional distinction between content words and function words (or open-class and closed-class items) relies on the stipulation that the former have their &amp;quot;own&amp;quot; meaning independent of the context in which they are used, whereas the latter assume meaning only in context. Then, content words are assigned to the realm of the lexicon, whereas function words are treated as a part of grammar.</Paragraph>
      <Paragraph position="1"> For dealing with discourse markers, we do not regard this distinction as particularly helpful, though. As we have illustrated above and will elaborate below, these words can carry a wide variety of semantic and pragmatic overtones, which render the choice of a marker meaningdriven, as opposed to a mere consequence of structural decisions. Furthermore, a number of lexical relations that are customary used to assign structure to the universe of &amp;quot;open class&amp;quot; lexical items, most prominently synonymy, plesionymy (&amp;quot;near-synonymy&amp;quot;), antonymy, hyponymy and polysemy, can be applied to discourse markers as well: * Synonymy: It can be argued that true synonyms do not exist at all. However, the German words obzwar and obschon (both more formal variants of obwohl = although) certainly come very close to being synonymous.</Paragraph>
      <Paragraph position="2"> * Plesionymy: although and though, according to Martin \[1992\], differ in formality; although and even though differ in terms of emphasis.</Paragraph>
      <Paragraph position="3"> * Antonomy: if/unless, according to Barker \[1994\], have opposite polarity, as in He will not attend unless he finishes his paper vs.</Paragraph>
      <Paragraph position="4"> He will attend if he finishes his paper.</Paragraph>
      <Paragraph position="5"> * Hyponomy: Some markers are more specific than others; recall the example of but given above. Knott and Mellish \[1996\] deal with the issue of &amp;quot;taxonomizing&amp;quot; discourse markers.</Paragraph>
      <Paragraph position="6"> * Polysemy: Other than being more or less specific, some markers can signal quite different relations; e.g., while can be used for TEMPORAL CO-OCCURRENCE, and also for CONTRAST.</Paragraph>
      <Paragraph position="7"> Accordingly, we propose that the proper place for describing discourse markers is a dedicated lexicon that provides a classification of their syntactic, semantic and pragmatic features and characterizes the relationships between similar markers. To this end, our group is developing a Discourse Marker LEXicon (DiMLex), which aims at assembling the various information associated with markers and describing it on a uniform level of representation. Our initial focus is on German, but English will also be a target language.</Paragraph>
    </Section>
    <Section position="2" start_page="1238" end_page="1239" type="sub_section">
      <SectionTitle>
2.2 Methodology
</SectionTitle>
      <Paragraph position="0"> Methodological considerations pertain to the two tasks of determining the set of words we regard as discourse markers and thus are to be included in the lexicon, and determining the lexical entries for these words.</Paragraph>
      <Paragraph position="1"> Finding the &amp;quot;right&amp;quot; set of discourse markers is not an easy task, since the common lexicographic practice of taking part of speech as the primary criterion for inclusion or exclusion does not apply. Knott and Mellish \[1996\] provide an apt summary of the situation. Their 'test for relational phrases' is a good start, but geared towards the English language (we are investigating German as well), and furthermore it catches only items relating clauses; in Despite the heavy rain, we went for a walk it would not detect a cue phrase.</Paragraph>
      <Paragraph position="2"> To arrive at a more comprehensive set, we began by consulting standard grammars such' as Quirk et al. \[1972\] and Helbig and Buscha \[1991\], which provide descriptions of function words grouped according to semantic class -but these are far from &amp;quot;complete&amp;quot;. A very good source for German is \[Brausse et al. in prep.\], which investigates a huge set of connectives from a grammatical viewpoint.</Paragraph>
      <Paragraph position="3"> As for determining lexical descriptions, the research literature offers a large number of helpful, even though quite heterogeneous, sources. There are several detailed studies of individual groups of markers, such as \[Vander Linden, Martin 1995\] for PURPOSE markers. Besides, the Linguistics literature offers fine-grained analyses of individual markers, which are far too numerous to list. We are drawing upon all these sources, trying to place them in a single unified framework. The overall goal can be characterized as the aim to synthesize two strands of re- null search that so far are rather disconnected: * &amp;quot;Top-down&amp;quot;: Text linguistics considers markers as a means to signal coherence, and provides us with insights on the semantic and pragmatic properties of marker classes.</Paragraph>
      <Paragraph position="4"> * &amp;quot;Bottom-up&amp;quot;: Grammars as well as the linguistic research literature provide syntactic, semantic and stylistic properties of individual markers, comparative studies of related markers, etc.</Paragraph>
    </Section>
    <Section position="3" start_page="1239" end_page="1239" type="sub_section">
      <SectionTitle>
2.3 The lexicon
</SectionTitle>
      <Paragraph position="0"> Although our classification of lexical features is still under development, we give here a tentative list of such features in order to illustrate the range of phenomena under consideration. The list is loosely ordered from syntactic to semantic and pragmatic features; for now, we do not explicitly assign such categories.</Paragraph>
      <Paragraph position="1"> The part of speech of a marker (conjunctive, subordinating conjunction, coordinating conjunction, preposition) determines the possibilities of positioning the marker within the constituent: conjunctives (especially the German 'Konjunktionaladverbien') can float to various positions, whereas the positions of others are fixed. The linear order of the conjuncts is fixed for some markers and flexible for others; this is independent of the aforementioned two features.</Paragraph>
      <Paragraph position="2"> Some markers show a specific behavior towards negation, e.g., the German sondern (which corresponds to certain uses of but) requires an explicit negation in the antecedent clause. Some markers impose constraints on tense and aspect of the clauses, either by requiring specific temporal/aspectual attributes in one clause, or by constraining the relationship between the two conjuncts (e.g., after).</Paragraph>
      <Paragraph position="3"> Several grammars suggest classifications of markers according to the semantic relation they express: adversative, alternative, substitution, causal, conditional, etc. Within these groups, some markers exhibit opposite polarity, i.e., have an incorporated negation or not (e.g., if versus unless). Commentability is a feature that often distinguishes a single marker within a semantic class in that it can be negated or focused on by scalar particles (e.g., in German, the causal weil is commentable, whereas denn is not).</Paragraph>
      <Paragraph position="4"> Moving towards pragmatics, the intention behind using a marker can vary. A well-known example is the contrast between German aber and sondern (in English, they both correspond to but), where the former merely states a contrast, whereas the latter corrects an assumption on the hearer's side (e.g., \[Helbig, Buscha 1991\]). Another dimension concerns the presuppositions associated with markers; a well-known case is the contrast between because and since, where only the latter marks the subsequent proposition as given. The German CAUSE markers well and denn differ in terms of the illocutions they connect: the former applies to propositions, the latter to epistemic judgements \[Brausse et al., in prep.\]. Certain very similar markers differ only stylistically. One German example was given above, and another one is the English notwithstanding, which is more formal than despite but moreover is more flexible in positioning, as it can be postponed.</Paragraph>
      <Paragraph position="5"> The final but crucial feature to be mentioned here is the discourse relation expressed by a marker. RST \[Mann, Thompson 1988\] offers an inspiring theory of such relations, but we do not fully subscribe to this account. Rather, we think that the relationship between semantic relations (see above) and pragmatic ones needs to be clarified (e.g., lasher 1993\]), which can be done by teasing apart the various dimensions incoporated in RST's definitions, for example in the spirit of Sanders et al. \[1992\].</Paragraph>
      <Paragraph position="6"> Once the range of dimensions has been described, we will deal with questions of representation; we envisage using some inheritance-based formalism that allows for a compact representation of individual descriptions, hyponymic relations between them, and polysemous entries.</Paragraph>
    </Section>
  </Section>
  <Section position="5" start_page="1239" end_page="1240" type="metho">
    <SectionTitle>
3 Using DiMLex in text generation
</SectionTitle>
    <Paragraph position="0"> Present text generation systems are typically not very good at choosing discourse markers. Even though a few systems have incorporated some more sophisticated mappings for specific relations (e.g., in DRAFTER \[Paris et al. 1995\]), there is still a general tendency to treat discourse marker selection as a task to be performed as a &amp;quot;side effect&amp;quot; by the grammar, much like for other function words such as prepositions.</Paragraph>
    <Paragraph position="1">  To improve this situation, we propose to view discourse marker selection as one subtask of the general lexical choice process, so that -- to continue the example given above -- one or another form of CONCESSION can be produced in the light of the specific utterance parameters and the context. Obviously, marker selection also includes the decision whether to use any marker at all or leave the relation implicit (e.g., \[Di Eugenio et al. 1997\]). When these decisions can be systematically controlled, the text can be tailored much better to the specific goals of the generation process.</Paragraph>
    <Paragraph position="2"> The generation task imposes a particular view of the information coded in DiMLex: the entry point to the lexicon is the discourse relation to be realized, and the lookup yields the range of alternatives. But many markers have more semantic and pragmatic constraints associated with them, which have to be verified in the generator's input representation for the marker to be a candidate. Then, discourse markers place (predominantly syntactic) constraints on their immediate context, which affects the interactions between marker choice and other realization decisions. And finally, markers that are still equivalent after evaluating these constraints are subject to a choice process that can utilize preferential (e.g. stylistic) criteria. Therefore, under the generation view, the information in DiMLex is grouped into the following three classes: -- Applicability conditions: The necessary conditions for using a discourse marker, i.e., the features or structural configurations that need to be present in the input specification.</Paragraph>
    <Paragraph position="3"> -- Syntagmatic constraints: The constraints regarding the combination of a marker and the neighbouring constituents; most of them are syntactic and appear at the beginning of the list given above (part of speech, linear order, etc.). -- Paradigmatic features: Features that label the differences between similar markers sharing the same applicability conditions, such as stylistic features and degrees of emphasis.</Paragraph>
    <Paragraph position="4"> Very briefly, we see discourse marker choice as one aspect of the sentence planning task (e.g., \[Wanner, novy 1996\]). In order to account for the intricate interactions between marker choice and other generation decisions, the idea is to employ DiMLex as a declarative resource supporting the sentence planning process, which comprises determining sentence boundaries and sentence structure, linear ordering of constituents (e.g., thematizations), and lexical choice. All these decisions are heavily interdependent, and in order to produce truly adequate text, the various realization options need to be weighted against each other (in contrast to a simple, fixed sequence of making the types of decisions), which presupposes a flexible computational mechanism based on resources as declarative as possible. This generation approach is described in more detail in a separate paper \[Grote, Stede 1998\].</Paragraph>
  </Section>
  <Section position="6" start_page="1240" end_page="1241" type="metho">
    <SectionTitle>
4 Using DiMLex in text
</SectionTitle>
    <Paragraph position="0"> understanding In text understanding, discourse markers serve as cues for inferring the rhetorical or semantic structure of the text. In the approach proposed by Marcu \[1997\], for example, the presence of discourse markers is used to hypothesize individual textual units and relations holding between them. Then, the overall discourse structure tree is built using constraint satisfaction techniques. For tasks of this kind, DiMLex can supply the set of cue words to be looked for and support the initial disambiguation of cues in the text. Depending on the depth of the syntactic and semantic analysis carried out by the text understanding system, different features provided by DiMLex can be taken into account. Certain structural configurations can be tested without any deep understanding; for instance, the German marker w~ihrend is generally ambiguous between a CONTRAST and a TEMPORALCOOCCURRENCE reading, but when followed by a noun phrase, only the latter reading is available (wiihrend corresponds not only to the English while but also to during).</Paragraph>
    <Paragraph position="1"> Similarly, we envisage applications of DiMLex for dialogue processing. For example, within the VERBMOBIL project, Stede and Schmitz \[1997\] have analysed the various pragmatic functions that German discourse particles fulfill in dialogue; many of these particles are discourse markers, and DiMLex can provide valuable information for their disambiguation, which in turn facilitates the recognition of underlying speech acts.</Paragraph>
  </Section>
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