a man - machine dialogue system employing an interactive computer system comprising an input device for receiving input from the user . an output device generating output to the user . there is provided an object system which is an information source or store , or a command and control device . the system has a dialogue manager , which orchestrates the dialogue between the object system and user dependent upon a dialogue specification . the specification employs a set of augmented transition networks , through which are propagated one or more tokens , the token comprising a set of fields which together define the current state of belief of the dialogue controller , dependent upon the preceding user - system interchanges and information obtained from the object system .