5. The method as set forth in claim 4 , wherein the maintaining a non-vocabulary word index includes: storing for each word of the non-vocabulary word index (i) an indication of the number of documents containing the word, (ii) an indication of the category of each document containing the word, and (iii) a number of occurrences of the word in each document containing the word.
4. The method as set forth in claim 3 , wherein the updating further includes: maintaining a non-vocabulary word index containing words that have occurred in the collection of documents, the identifying of the new vocabulary word referencing the non-vocabulary word index.