A transaction system has machine recognition of speech. It has dialogue control fed by the recognition, and speech generation fed by the dialogue control for outputting question and verifier statements from a repertoire set. A human-machine dialogue is executed until the dialogue control has recognized a viable transaction formulation with a plurality of user-provided slot fillers to specify the transaction. Dialogue control builds a directed and loopless status graph with nodes that each have their own slot filler and associated metric, and are interrelated through logic relations. The building can amend a node's metric and under control of conflict detection or lowering of a particular node's metric, discard the node in question and its filler, including of derived nodes and also of one-to-one derival nodes of the discarded node. A subset of problems is detected for a particular item outputted by the speech generation for ascertaining a particular slot filler, as combined with a subsequent unsatisfactory recognition. Then the speech generation outputs a further question statement for ascertaining the latter slot filler.