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<Paper uid="W00-1415">
  <Title>An Empirical Analysis of Constructing Non.restrictive NP Modifiers to Express Semantic Relations</Title>
  <Section position="3" start_page="0" end_page="108" type="intro">
    <SectionTitle>
1 Introduction
</SectionTitle>
    <Paragraph position="0"> To produce natural language text, an NLG system must be able to choose among possible paraphrases one that satisfies the highest number of constraints in a certain context. Paraphrases can use various constructions, for example, using nominalisation instead of a clause for event representation. We are particularly interested in the use of non-restrictive (NR) modifiers within a referring expression to express certain semantic relationQ other than objectattribute elaboration (in the sense defined in (Mann and Thompson, 1987)), for instance, causal relations, which are normally expressed by separate clauses connected by cue phrases (Knott, 1996) such as 'because '.</Paragraph>
    <Paragraph position="1"> &amp;quot;A non-restrictive component gives additional information to a head that has already been viewed as unique or as a member of a class that has been independently identified,-mud therefoee is not' essml; tial for the identification of the head&amp;quot; (Quirk et al., 1985). This definition can be extended to account for modifiers of not only definite referring expressions, but also definite and indefinite NPs of various types. In this paper, an NR modifier refers to any NP modifying component that is not essential for identifying the object denoted by the head, including all modifiers of an NP that does not intend to identify (e.g. indefinite referring expressions and predicative phrases) (Kronfeld, 1990). Our discussion focuses on definite referring expressions including proper-names because of the dominance of such examples in our corpus. However, we would expect no difficulty in applying our observation to other types of NPs.</Paragraph>
    <Paragraph position="2"> The semantic roles of NR modifiers, in particular NR clauses, are mentioned in many grammar and linguistics books. Quirk et al. (1985) point out that an NR clause in a referring expression is usually neutral in its semantic role (i.e. it provides descriptive information about its head), but sometimes it can contribute to the semantics of the main clause in a variety of ways. They summarise three types of semantic relations that can be expressed by an NR clause (examples are given in Figure 1):  (r) Causal, where the situation in the main clause is caused by that in the NR clause, e.g. (la).</Paragraph>
    <Paragraph position="3"> (r) Temporal, where the two clauses form a time sequence, e.g. (lb).</Paragraph>
    <Paragraph position="4"> (r) Circumstantial, where the NR clause sets a tem null poral or spatial framework for interpreting the main clause, e.g. (lc).</Paragraph>
    <Paragraph position="5"> Halliday (1985) mentions that a subordinate clause can elaborate a part of its primary clause through restating, clarifying, refining or adding a descriptive attribute or comment (see (2) of Figure 1). Halliday's notion of elaboration is much more general than that in other coherence theories like RST (Maim &amp;quot;andThompson; t987), and :the rdation expressed in (2) would not be treated as elaboration in most NLG systems.</Paragraph>
    <Paragraph position="6"> Similar phenomena were observed from the MUSE corpus 2, a corpus of museum exhibit labels, which l kVe are concerned with semantic (informational) relations in this paper* Argumentative {intentional) relations are beyond the scope of this paper.</Paragraph>
    <Paragraph position="7"> 2This corpus is collected and annotated for the GNOME project (Poesio, 2000), which aims at developing general algorithms for generating nominal expressions.</Paragraph>
    <Paragraph position="8">  (1) a.</Paragraph>
    <Paragraph position="9"> b.</Paragraph>
    <Paragraph position="10"> C.</Paragraph>
    <Paragraph position="11">  He sent ahead the se,yeant, who was the most experienced scout in the company. In 1960 he came to ,London4 .:wh.are. :he :haa~lived * ever ~in~ze. The boy, who had his satchel trailing behind him, ran past.  (2) Inflation, which was necessary for the system, became also &amp;quot;lethal. (3) In spite of his French name, Martin Carlin was born in Germany and emigrated to Paris to become an ebeniste.</Paragraph>
    <Paragraph position="13"> describe museum objects on display. For example, in (3) of Figure 1, the modifier French is not for identifying the name, but for establishing a concession relation between the main proposition and the subordinate phrase to increase the reader's positive regard for where Martin Carlin was born.</Paragraph>
    <Paragraph position="14"> For the convenience of discussion, we define some terminology to be used throughout the paper: An NR construction/sentence : a sentence that has a main clause and a subordinate NR modifier attached to one of its NPs (e.g. (4b) of Figure 2).</Paragraph>
    <Paragraph position="15"> A hypotactic construction/sentence : a sentence that has a main clause and a dependent clause, connected by a cue phrase. This is a common way of expressing semantic relations such as causality (e.g. (4a) of Figure 2). In this syntactic category, we single out a subclass of sentences according to one possible semantic connection between the two clauses. It is defined below.</Paragraph>
    <Paragraph position="16"> An elaboration realisation : a type of hypotactic construction where one clause elaborates the semantics of the other. We take cue phrases &amp;quot;as for&amp;quot; or &amp;quot;what is more&amp;quot; to signal elaboration relations 3 .</Paragraph>
    <Paragraph position="17"> Previous research in NLG mainly focuses on using NR constructions to realise elaboration relations but not other semantic relations (e.g. (Scott and de Souza, 1990) and (Hovy, 1993)). The NR moditier usually adds a descriptive attribute to the object denoted by the head.</Paragraph>
    <Paragraph position="18"> The linguistic research suggests for an NLG system the possibility to express certain semantic relacue phrases in most cases, and therefore could avoid using cues too heavily., This could be a better realisation under certain circumstances. Secondly, an NR construction enables a wider range of relations (especially those that are preferred to be expressed implicitly) to be selected for text structuring because the corresponding syntactic option is available.</Paragraph>
    <Paragraph position="19"> To understand how to enable an NLG system to generate such modifiers, we are faced with two questions, which are not answered by linguistic research:  1. Can this type of modifier be identified by human subjects, i.e. can humans tell the difference between different NP modifier uses? 2. Under what circumstances can an NR construc- null tion be used in substitution of a hypotactic construction without changing the meaning dramatically and how close are the meanings conveyed by the two representations? An NLG system must come up with some solutions, simple or complex, to these two questions in order to choose among paraphrases. In this paper, we use cue phrases ms a signal of semantic relations rather than try to identify the relations directly. We describe systematically controlled experiments aimed at finding out the factors related to the generation of this type of modifier in referring expressions. The result is intended to be reliable enough to be used by NLG systems in generating descriptive text.</Paragraph>
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