“…The ability to reason about plans has been used in widely diverse tasks, such as resolving referring expressions (Grosz, 1977), building a psychological model (Schmidt, Sridharan & Goodson, 1978) and understanding stories (Schank & Abelson, 1977;Wilensky, 1983;Charniak & Goldman, 1993). The incorporation of plan recognition capabilities into computerized information providers has enabled a range of cooperative behaviours, such as supplying more information than what is explicitly requested , responding to ill-formed queries (Carberry, 1988), and understanding (a) indirect speech acts (Perrault & Allen, 1980), (b) inter-sentential ellipsis (Carberry, 1985;Litman, 1986), (c) queries based on invalid plans (Pollack, 1990), and (d) sub-dialogues entered into to debug or correct plans (Sidner, 1985;Litman & Allen, 1987). More recently, there has been research to develop plan recognition systems that ar e capable of considering multiple alternatives in order to recognize intentions that are developed and revised over multiple utterances (Kautz & Allen, 1986;Carberry, 1990;Raskutti & Zukerman, 1991;Appelt & Pollack, 1992;Charniak & Goldman, 1993).…”