Anchoring a Lexicalized Tree-Adjoining Grammar for Discourse

Anchoring a Lexicalized Tree-Adjoining Grammar for Discourse Bonnie Lynn Webber and Aravind K. Joshi Department of Computer & Information Science University of Pennsylvania Philadelphia PA USA 19104-6389 [bonnie, joshi]~central.cis.upenn.edu Abstract We here explore a "fully" lexicalized Tree-Adjoining Grammar for discourse that takes the basic ele- ments of a (monologic) discourse to be not simply clauses, but larger structures that are anchored on variously realized discourse cues. This link with intra-sentential grammar suggests an account for dif- ferent patterns of discourse cues, while the differ- ent structures and operations suggest three sepa- rate sources for elements of discourse meaning: (1) a compositional semantics tied to the basic trees and operations; (2) a presuppositional semantics carried by cue phrases that freely adjoin to trees; and (3) general inference, that draws additional, defeasible conclusions that flesh out what is conveyed compo- sitionally. 1 Introduction In the past few years, researchers interested in ac- counting for how elements combine in a discourse, have taken to using the adjoining operation found in Tree-Adjoining Grammar (TAG) (Gardent, 1994; Gardent, 1997; Polanyi and van den Berg, 1996; Schilder, 1997; van den Berg, 1996; Webber, 1991). More recently, Cristea and Webber (1997) have ar- gued that a Tree-Adjoining Grammar for discourse would also need the substitution operation found in a lexicalized TAG (Schabes, 1990). Here we move further and explore a fully lexicalized TAG for dis- course, allowing us to examine how the insights of lexicalized grammars - that the basic elements of a clause are not simply words, but structures that re- flect a word's role and syntactic/semantic scope - carry over to discourse. We show how this suggests explanations for such phenomena as the following: • that arguments of a coherence relation can be stretched "long distance" by intervening mate- rial; • that multiple discourse cues can appear in a sin- gle sentence or even a single clause; • that when discourse cues appear in the middle of clauses, they contribute to coherence in more specific ways; • that coherence relations can vary in how and when they are realized lexically. One way of understanding the current work is that it extrapolates from lexically-based views of how structure and meaning are associated within a sentence to how aspects of discourse structure and meaning might be associated in similar ways. While the idea that discourse-level mechanisms might re- semble intra-sentential mechanisms has long been an undercurrent within discourse research, we have come to believe that the framework of lexicalized grammar can be effecti'.'ely used to demonstrate the validity of this intuition. While we present the ideas in terms of one well-known formalism - Lexicalized TAG - other lexicalized formalisms such as CCG (Steedman, 1996b) might prove equally useful for expressing the same theoretical insights and imple- menting them for discourse generation and/or inter- prey ation. A superficial reading of the current proposal might suggest that it ismerely a simple embedding of RST (Mann and Thompson, 1988) in TAG. That would be incorrect. First, the primary feature of …

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