8. The method of claim 1 , wherein selecting the plurality of documents responsive to the query comprises: identifying an incomplete phrase in the query; replacing the incomplete phrase with a phrase extension, wherein the phrase extension of the incomplete phrase is a super-sequence of the incomplete phrase that begins with the incomplete phrase, and wherein the incomplete phrase predicts the phrase extension; and selecting documents from the document collection containing the phrase extension, the phrase extension being one of the second phrases.
8. The method of claim 1 , wherein selecting the plurality of documents responsive to the query comprises: identifying an incomplete phrase in the query; replacing the incomplete phrase with a phrase extension, wherein the phrase extension of the incomplete phrase is a super-sequence of the incomplete phrase that begins with the incomplete phrase, and wherein the incomplete phrase predicts the phrase extension; and selecting documents from the document collection containing the phrase extension, the phrase extension being one of the second phrases. 9. The method of claim 8 , wherein the incomplete phrase predicts the phrase extension based on a measure of an actual frequency of the phrase extension being greater than an expected frequency of the phrase extension in the document collection.