44. A 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 from a user; identify a multiple word phrase in the query; identify a phrase extension of the identified phrase, wherein the phrase extension of the identified phrase is a sequence of words that begins with the identified phrase but is longer than the identified phrase, and wherein the identified phrase predicts the phrase extension based on a measure of an actual co-occurrence rate of the phrase extension and the identified phrase exceeding an expected co-occurrence rate of the phrase extension and the identified phrase in the document collection; and select documents from the document collection containing the phrase extension.
44. A 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 from a user; identify a multiple word phrase in the query; identify a phrase extension of the identified phrase, wherein the phrase extension of the identified phrase is a sequence of words that begins with the identified phrase but is longer than the identified phrase, and wherein the identified phrase predicts the phrase extension based on a measure of an actual co-occurrence rate of the phrase extension and the identified phrase exceeding an expected co-occurrence rate of the phrase extension and the identified phrase in the document collection; and select documents from the document collection containing the phrase extension. 51. The system of claim 44 , 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 of 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.