1. A computer-implemented expert system including: a memory for: storing a plurality of hypotheses, where the hypotheses are arranged in at least a first and second disjoint group of hypotheses, wherein the first group of hypotheses includes hypotheses that may be taken by the system and the second group includes hypotheses that may not be taken by the system; and storing questions for rejecting hypotheses of the second group; an output for supplying questions from the memory; an input for receiving initial data and answers to outputted questions; a processor programmed to: select questions from the stored questions for those hypotheses from the second group that are possible in dependence on the initial data; determine from at least one answer received in response to outputting the selected questions whether at least one of the hypotheses of the second group is possible by comparing received answers to a predetermined answer sequence stored in or derivable from the memory; and in response to determining that no hypothesis of the second group is possible, supply via the output a most likely hypothesis of the first group in dependence on at least the initial data, wherein the expert system is used for medical diagnosis, the initial data includes a health complaint, the second group of hypotheses containing medical diagnoses that require attention of a medical expert, the first group of hypotheses containing medical diagnoses that do not require any medical attention or can be cured or treated without assistance of a medical expert, and the processor is programmed to select the questions in dependence on the health complaint.
1. A computer-implemented expert system including: a memory for: storing a plurality of hypotheses, where the hypotheses are arranged in at least a first and second disjoint group of hypotheses, wherein the first group of hypotheses includes hypotheses that may be taken by the system and the second group includes hypotheses that may not be taken by the system; and storing questions for rejecting hypotheses of the second group; an output for supplying questions from the memory; an input for receiving initial data and answers to outputted questions; a processor programmed to: select questions from the stored questions for those hypotheses from the second group that are possible in dependence on the initial data; determine from at least one answer received in response to outputting the selected questions whether at least one of the hypotheses of the second group is possible by comparing received answers to a predetermined answer sequence stored in or derivable from the memory; and in response to determining that no hypothesis of the second group is possible, supply via the output a most likely hypothesis of the first group in dependence on at least the initial data, wherein the expert system is used for medical diagnosis, the initial data includes a health complaint, the second group of hypotheses containing medical diagnoses that require attention of a medical expert, the first group of hypotheses containing medical diagnoses that do not require any medical attention or can be cured or treated without assistance of a medical expert, and the processor is programmed to select the questions in dependence on the health complaint. 10. An expert system as claimed in claim 1 , wherein the processor is programmed to transfer control to a human expert.