Introduction One of the central concerns of a theory of pra~atics is to explain what actions language users p e r f o r m by making utterances. This concern is also relevant to the designers of conversational language understanding systems, especially those intended to cooperate with a user in t h e e x e c u t i o n o f some t a s k ( e . g . , t h e Computer C o n s u l t a n t t a s k d i s c u s.