Tense Trees as the “Fine Structure” of Discourse

TENSE TREES AS THE "FINE STRUCTURE" OF DISCOURSE Chung Hee Hwang &: Lenhart K. Schubert Department of Computer Science University of Rochester Rochester, New York 14627, U. S. A. {hwang, schubert }@cs. rochester, edu ABSTRACT We present a new compositional tense-aspect deindex- ing mechanism that makes use of tense trees as com- ponents of discourse contexts. The mechanism allows reference episodes to be correctly identified even for embedded clauses and for discourse that involves shifts in temporal perspective, and permits deindexed logical forms to be automatically computed with a small num- ber of deindexing rules. 1 Introduction Work on discourse structure, e.g., [Reichman, 1985; Grosz and Sidner, 1986; Allen, 1987], has so far taken a rather coarse, high-level view of discourse, mostly treating sentences or sentence-like entities ("utterance units, .... contributions," etc.) as the lowest-level dis- course elements. To the extent that sentences are ana- lyzed at all, they are simply viewed as carriers of certain features relevant to supra-sentential discourse structure: cue words, tense, time adverbials, aspectual class, into- national cues, and others. These features are presumed to be extractable in some straightforward fashion and provide the inputs to a higher-level discourse segment analyzer. However, sentences (or their logical forms) are not in general "flat," with a single level of structure and fea- tures, but may contain multiple levels of clausal and ad- verbial embedding. This substructure can give rise to arbitrarily complex relations among the contributions made by the parts, such as temporal and discourse rela- tions among subordinate clausal constituents and events or states of affairs they evoke. It is therefore essen- tial, in a comprehensive analysis of discourse structure, that these intra-sentential relations be systematically brought to light and integrated with larger-scale dis- course structures. Our particular interest is in tense, aspect and other indicators of temporal structure. We are developing a uniform, compositional approach to interpretation in which a parse tree leads directly (in rule-to-rule fash- ion) to a preliminary, indezical logical form, and this LF is deindezed by processing it in the current context (a well-defined structure). Deindexing simultaneously transforms the LF and the context: context-dependent constituents of the LF, such as operators past, pres and perf and adverbs like today or earlier, are replaced by explicit relations among quantified episodes; (anaphora are also deindexed, but this is not discussed here); and new structural components and episode tokens (and other information) are added to the context. This dual transformation is accomplished by simple recur- sive equivalences and equalities. The relevant context structures are called tense trees; these are what we pro- pose as the "fine structure" of discourse, or at least as a key component of that fine structure. In this paper, we first review Reichenbach's influen- tial work on tense and aspect. Then we describe tem- poral deindexing using tense trees, and extensions of the mechanism to handle discourse involving shifts in temporal perspective. 2 Farewell to Reichenbach Researchers concerned with higher-level discourse struc- ture, e.g., Webber [1987; 1988], Passonneau [1988] and Song …

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