Mark G. Core and Lenhart K. Schubert
Mixed-initiative dialogs often contain interruptions in phrase structure such as repairs and backchannel responses. Phrase structure as traditionally defined does not accommodate such phenomena, so it is not surprising that phrase structure parsers are ill-equipped to handle them. This paper presents metarules that specify, how the instantiations of phrase structure rules may be restarted or interrupted, with allowance for interleaved speech. In the case of interleaved speech or backchannel responses, tlle metarules allow syntactically separate constituents to interleave or to straddle each other. In the case of repairs, the metarules operate on the reparandum (what is being repaired) and alteration (the correction) to build parallel phrase structure trees: one with the reparandum and one with the alteration. Consider the partial utterance, take the ban- the oranges. The repair metarule would build two VPs, one being take the ban- and the other being take the oranges. The introduction of metarules simplifies the notion of an utterance since a sentence interrupted by an acknowledgment such as okay can stiU be treated as a single utterance formed around the interrupting acknowledgment. Together, metarules and phrase structure rules specify the structures that should be accommodated by a parser for mixed initiative dialogs. A dialog parser should also maintain a dialog chart that stores the results of syntactic and semantic analysis of all the dialog seen so far. This dialog chart will be a shared resource eliminating the nced for maintenance of a separate representation of dialog structure by a dialog manager.