5. The method of claim 1 wherein the initially processing comprises generating a list of the features present in the initial documents, and wherein the subsequently processing includes adding new features to the list which occur in the subsequent documents but do not occur in the initial documents.
5. The method of claim 1 wherein the initially processing comprises generating a list of the features present in the initial documents, and wherein the subsequently processing includes adding new features to the list which occur in the subsequent documents but do not occur in the initial documents. 7. The method of claim 5 wherein the list of the features indicates numbers of occurrences of respective ones of the features in the initial documents, and further comprising, after the initially identifying, determining a period of time which has passed since one of the initial documents was accessed, and further comprising decrementing one of the numbers of occurrences for one of the features which occurs in the one of the initial documents as a result of the period of time exceeding a threshold.