Mixed-initiative dialogs often contain interruptions in phrase structure such as repairs and backchannel responses. Phrase structure as traditionally de ned 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, the 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- um the oranges. The repair metarule would build two VPs, one being take the ban- and
Mark G. Core, Lenhart K. Schubert