5. The apparatus according to claim 1 including estimating means responsive to the processing means for estimating a maximum probability for a second document different from the first document based on a partially calculated probability for the second document and an assumption that the representations in the second document match the concepts of the query for which probabilities have not been calculated, the processing means being responsive to the estimating means to calculate partial probabilities that representations in the second document match concepts of the query until either the estimated maximum probability does not at least equal the threshold or the probability is calculated for all the concepts in the query.
5. The apparatus according to claim 1 including estimating means responsive to the processing means for estimating a maximum probability for a second document different from the first document based on a partially calculated probability for the second document and an assumption that the representations in the second document match the concepts of the query for which probabilities have not been calculated, the processing means being responsive to the estimating means to calculate partial probabilities that representations in the second document match concepts of the query until either the estimated maximum probability does not at least equal the threshold or the probability is calculated for all the concepts in the query. 6. The apparatus according to claim 5 wherein the output means includes a result list ranking the identified documents in probability order, the threshold setting means being responsive to the result list to reset the probability threshold equal to the probability of the document lowest on the result list.