Reference Hashed

Reference Hashed Frank Schilder Department for lnformatics University of Hamburg Vogt-K611n-Str. 30 22527 Hamburg, Germany schi Ider@ in formatik, uni - hamburg, de Abstract This paper argues for a novel data structure for the representation of discourse referents. A so-called hashing list is employed to store discourse referents according to their grammatical features. The ac- count proposed combines insights from several the- odes of discourse comprehension. Segmented Dis- course Representation Theory (Asher, 1993) is en- riched by the ranking system developed in center- ing theory (Grosz et al., 1995). In addition, a tree logic is used to represent underspecification within the discourse structure (Schilder, 1998). 1 Introduction Discourse referents are represented quite differently by current diso~urse theories. Discourse Represen- tation Theory (DRT), for example, employs a rather unstructured data structure for the domain of dis- course referents: a set (Kamp and Reyle, 1993). A DRT-implementation by Asherand Wada (1988), however, employs a more complex data type: a tree representation. In further work by Asher (i 993) ref- erents are grouped together into segments depend- ing on Ihe discourse structure. His Segmented DRT (SDRT) uses a tree-like representation for the dis- course sUuctui~ I Centering Theory (CD proposes a//st structure for the entities one preferably refers to in subse- quent sentences. In order to cover coreference over discourse segments the centering model was ex- tended by a stack mechanism (Grosz and Sidner, 1986). Recently, these data structures have been criticized by Walker (1998), because they seem to be too restrictive. She proposes a cache storage for the referenis in the focus of attention. ! propose instead a novel data structure for the representation of discourse referents. A/rushing list tSimilarly, Rhetorical Structure Theory (RST) makes use of a tree representation (Mann et aL,1988). 100 is used to distinguish between the different types of referents according to their grammatical features wrt. number or gender. This list structure is further- more combined with a hierarchical tree structure. The remaining pan of the paper is organised as follows. Section 2 introduces the main claims made by past theories. Focusing on SDRT and CT, I will highlight the (dis.-...) advantages of these two ap- proaches. Section 3 provides the reader with an introduction to hashing lists and how they can be used for linguistic dam. Section 4 discusses how the different advantages of former approaches can be combined. First, DRT will be amended by us- ing a hashing list for the discourse referents instead of a simple set. Second, the centering model will be applied to the representation gained. Finally, the shortcomings of a flat representation are presented and the introduction of discourse segments is dis- cussed. Subsequently, section 5 describes a detailed formalisation of one example sequences by the rep- resentation proposed, before section 6 concludes. 2 Background It has been commonly agreed that a discourse is hierarchically organised. However. this is already the lowest common denominator among current ap- proaches to discourse grammars and text compre- hension. There is a wide range of views …

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