Priority Union and Generalization in Discourse Grammars

We describe an implementation in Carpenter's typed feature formalism, ALE, of a discourse grammar of the kind proposed by Scha, Polanyi, et al. We examine their method for resolving parallelism-dependent anaphora and show that there is a coherent feature-structural rendition of this type of grammar which uses the operations of prwrity union and generalization. We describe an augmentation of the ALE system to encompass these operations and we show that an appropriate choice of definition for priority union gives the desired multiple output for examples of vP-ellipsis which exhibit a strict/sloppy ambiguity. 1 Discourse Grammar Working broadly within the sign-based paradigm exemplified by HPSG (Pollard and Sag in press) we have been exploring computational issues for a discourse level grammar by using the ALE system (Carpenter 1993) to implement a discourse grammar. Our central model of a discourse grammar is the Linguistic Discourse Model (LDM) most often associated with Scha, Polanyi, and their co-workers (Polanyi and Scha 1984, Scha and Polanyi 1988, Priist 1992, and most recently in Priist, Scha and van den Berg 1994). In LDM rules are defined which are, in a broad sense, unification grammar rules and which combine discourse constituent units (DCUS). These are simple clauses whose syntax and underresolved semantics have been determined by a sentence grammar but whose fully resolved final form can only be calculated by their integration into the current discourse and its context. The rules of the discourse grammar act to establish the rhetorical relations between constituents and to perform resolution of those anaphors whose interpretation can be seen as a function of discourse coherence (as opposed to those whose interpretation relies on general knowledge). For illustrative purposes, we focus here on Prfist's rules for building one particular type of rhetorical relation, labelled "list" (Priist 1992). His central thesis is that for DCUs to be combined into a list they must exhibit a degree of syntactic-semantic parallelism and that this parallelism will strongly determine the way in which some kinds of anaphor are resolved. The clearest example of this is vP-ellipsis as in (la) but Priist also claims that the subject and object pronouns in (lb) and (lc) are parallelism-dependent anaphors when they occur in list structures and must therefore be resolved to the corresponding fully referential subject/object in the first member of the list. (1) a. Hannah likes beetles. So does Thomas. b. Hannah likes beetles. She also likes caterpillars. …