13. A method as claimed in claim 11 wherein the items are advertisements and the features are keywords to which the advertisements are subscribed.
13. A method as claimed in claim 11 wherein the items are advertisements and the features are keywords to which the advertisements are subscribed. 14. A method as claimed in claim 13 which further comprises receiving an advertisement having a plurality of subscribed keywords and using the data structure to generate additional keywords for subscription to that advertisement.