1. 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 selecting, by at least one processor of the computing system, documents from the document collection, wherein the selected documents contain the phrase extension.