25. The article of claim 19 wherein the initially processing generates a forward index comprising the initial representations, and the subsequently processing comprises adding the subsequent representations to the forward index.
25. The article of claim 19 wherein the initially processing generates a forward index comprising the initial representations, and the subsequently processing comprises adding the subsequent representations to the forward index. 27. The article of claim 25 wherein the initially processing generates a reverse index which includes one list of the features and the initial documents which include respective ones of the features.