10. The method of claim 8 , wherein identifying the incomplete phrase in the query includes: identifying a candidate phrase in the query; comparing the candidate phrase to an incomplete phrase in a list of incomplete phrases that are present in documents of the document collection; and if the candidate phrase matches to the incomplete phrase, identifying the candidate phrase as an incomplete phrase and identifying a phrase extension associated with the incomplete phrase as the phrase extension of the incomplete phrase.
8. A computer-implemented method of selecting documents from a document collection in response to a query, the method comprising: receiving a query; identifying, by at least one processor of a computing system, an incomplete phrase in the query, wherein phrases predicted by the incomplete phrase in the document collection include only phrase extensions of the incomplete phrase; identifying, by at least one processor of the computing system, a phrase extension of the incomplete phrase, wherein the phrase extension of the incomplete phrase is a sequence of words that begins with the incomplete phrase but that is longer than the incomplete phrase, and wherein the incomplete phrase predicts the phrase extension based an actual co-occurrence rate of the phrase extension and the incomplete phrase in the document collection exceeding an expected co-occurrence rate of the phrase extension and the incomplete phrase in the document collection; and suggesting, by at least one processor of the computing system, the phrase extension to the user to use in the query.