  In this paper we present a formalization of the centering approach to
modeling attentional structure in discourse and use it as the basis for an
algorithm to track discourse context and bind pronouns. As described in Grosz,
Joshi and Weinstein (1986), the process of centering attention on entities in
the discourse gives rise to the intersentential transitional states of
continuing, retaining and shifting. We propose an extension to these states
which handles some additional cases of multiple ambiguous pronouns. The
algorithm has been implemented in an HPSG natural language system which serves
as the interface to a database query application.
