30. A system for selecting documents from a document collection in response to a query, the system comprising: one or more memory devices configured store executable instructions; and one or more processors configured to execute the stored instructions to cause the system to: receive a query; identify 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; identify 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 select documents from the document collection, wherein the selected documents contain the phrase extension.
30. A system for selecting documents from a document collection in response to a query, the system comprising: one or more memory devices configured store executable instructions; and one or more processors configured to execute the stored instructions to cause the system to: receive a query; identify 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; identify 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 select documents from the document collection, wherein the selected documents contain the phrase extension. 31. The system of claim 30 , wherein the expected co-occurrence rate of the phrase extension is a function of a number of documents in the document collection that include the identified phrase and a number of documents in the document collection that include the phrase extension, and the actual co-occurrence rate being a function of a number of times the phrase extension appears within a threshold number of words of the identified phrase in the document collection.