Embedding New Information into Referring Expressions

This paper focuses on generating referring expressions capable of serving multiple communicative goals. The components of a referring expression are divided into a referring part and a non-referring part. Two rules for the content determination and construction of the non-referring part are given, which are realised in an embedding algorithm. The significant aspect of our approach is that it intends to generate the non-referring part given the restrictions imposed by the referring part, whose realisation is, on the other hand, affected by the non-referring part. 1 Components of a Referring Expression The referring expression is a very important and complex construction in languages. It can serve multiple communicative goals including referring to an object, providing new information about it, and expressing the speaker's emotional attitude towards it (Appelt, 1985). Although a formal model of referring built within the framework of a general theory of speech acts and rationality is given in (Appelt and Kronfeld, 1987), and this can be used to explain how referring acts achieve multiple goals, there is a gap between the general model and the planning of the linguistic content of a referring expression. We divide the constituents in a referring expression I into two parts based on their communicative goals and the rules for their content determination and realisation. They are a re)"erring part , which intends to refer to an object and a non-re ferr ing part, which intends to provide additional new information about the object. For example, in "the actual writing style of Xuanzong, who was a we l l known ca l l igrapher" , the bold faced items belong to the referring part, and the underlined ones to the non-referring part. The division is a pragmatic one and the two parts are closely related to each other. On the one hand, the referring part puts both syntactic and semantic lOnly singular referring expressions that are primarily for referring to physical objects are considered here. constraints on the presenting of the non-referring part. The syntactic constraint concerus mainly the available syntactic slots around the head. The semantic constraint will be introduced in section 3. On the other hand, the possibility of adding a nonreferring part can make some realisations of a referent preferred over others. When generating referring expressions, multiple factors should be considered, which include Centering Theory (Grosz et al., 1995) and stylistic preferences such as avoiding too many repetitions. If we are to satisfy all constraints to some extent, we may need to consider more than one possible realisation of a referent, choosing among those that do not significantly affect the coherence of the text. Then one of the realisations that is most suitable for adding new information can be selected. A great amount of work has been done on generating various types of referring expressions, which addresses the referring part, while little has addressed the generation issues with respect to the other part, except that in (Scott and de Souza, 1990), the relation between embedding and rhetorical relations is discussed and several heuristics for combining sentences using embedding are given. But this is far from enough for generating an appropriate referring expression. 2 S y s t e m A r c h i t e c t u r e We design an algorithm to generate referring expressions consisting of both parts. The refmTing part is generated by the referring process (Dale, 1992), while the non-referring part is generated by a subtype of the aggregation process called embedding, which selects suitable facts and realises them as components within the structure of a referring expression. The algorithm fits into the text planner of ILEX (Oberlander et al., 1998). ILEX is an adaptive hypertext system generating museum object descriptions. I11 ILEX, pieces of domain knowledge that may be worth expressing in a text are represented as nodes and links in a graph called the Content Potential. Two kinds of nodes