An Architecture for Anaphora Resolution

An Architecture for Anaphora Resolution Elaine Rich Susann LuperFoy MCC, Advanced Computer Architecture Program 3500 West Balcones Center Drive Austin, TX 78759 ABSTRACT In this paper, we describe the pronominal anaphora resolution module of Lucy, a portable English understanding system. The design of this mo;clule was motivated by the observation that, al- though there exist many theories of anaphora resolu- tion, no one of these theories is complete. Thus we have implemented a blackboard-like architecture in which individual partial theories can be encoded as separate modules that can interact to propose can- didate antecedents and to evaluate each other's proposals. INTRODUCTION The Lucy system (Rich, 1987) is a prototype of a portable English front end for knowledge- based systems. The major components of Lucy are a syntax-based parser (Wittenburg, 1986), a semant=c translation system, a pronominal anaphora resolution system (which will be described in this paper) and a pragmatic proces- sor. The parser p..roduces as !ts output a feature graph that descnoes the syntacticpropenies ov the constituents of the sen(ence. The semantic translation system produces as its output a list of discourse referents and a set of assertions about them. The job of the anaphora resolution system is to augment this assertion set with ad- ditional assertions that describe coreference relations between discourse referents. Figure 1 shows the results of semantic processing and anaphora resolution for the simple discourse, "Dave created a file. He printed it." A DISTRIBUTED ARCHITECTURE Designing an anaphora resolution system is difficult because there exists no single, coherent theory upon which to build, even =f we restrict our attention to pronominal anaphora (which we will do throughout this paper). There do, however, exist many partial theories, 1 each of which accounts for a subset of the phenomena that influence the use and interpretation of pronominal anaphora. These partial theories range from purely syntactic ones (for example the simple rules of number and gender agreement) to highly semantic and pragmatic ones that account for focusing phenomena. If there were a single, complete theory, then it might be appropriate to implement it. If there were no theories at all, then an ad hoc im- p lementation might be the only alternative. But ecause there are partial theones but not a com- plete one, we have designed an architecture (patterned after the idea of a blackboard system (Erman, 1981)) that allows for a loosely coupled set of modules, each of which handles a subset of discourse phenomena by implementing a specific partial theory. These modules com- municate by proposing candidate antecedents and by evaluating each other's proposals. An oversight module, called the handler, mediates these communications and resolves conflicts among the modules. All the modules in this system share a com- mon representation of the current discourse. This representation is called the core discourse structure. It includes a list of the discourse referents that have so far been introduced. As- sociated with each such referent is a set of assertions, including syntactic facts about the use of the referent …

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