37. 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 suggest the phrase extension to the user to use in the query.
40. The system of claim 37 , wherein the suggested phrase extension is selected from a plurality of phrase extensions of the identified phrase, based on information gains of the plurality of phrase extensions given the identified phrase.