1. A computer-implemented method of selecting documents in a document collection in response to a query, the method comprising: receiving a query; identifying an incomplete phrase in the query, wherein other phrases predicted by the incomplete phrase in the document collection include only phrase extensions of the incomplete phrase; 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 based on a measure of an actual co-occurrence rate of the phrase extension and the incomplete phrase exceeding an expected co-occurrence rate of the phrase extension and the incomplete phrase in the document collection, the expected co-occurrence rate of the phrase extension and the incomplete phrase being a function of a plurality of occurrences of the phrase extension and the incomplete phrase in the document collection; selecting documents from the document collection containing the phrase extension; and storing the selected documents in a memory as part of a search result.