9. A system for modifying a query state using a query received from a user of a device having a processor, the system comprising: a query term identifying component configured to identify query terms in the query and in a follow-up query; a query state modification identifying component configured to: identify a query state modification set comprising at least two candidate query state modifications respectively having a query state modification probability, and from the query state modification set, identify a selected query state modification having a higher query state modification probability for the follow-up query than the query state modifications of other candidate query state modifications; and a query state modifying component configured to modify the query state according to the selected query state modification and the query terms identified in the follow-up query.
9. A system for modifying a query state using a query received from a user of a device having a processor, the system comprising: a query term identifying component configured to identify query terms in the query and in a follow-up query; a query state modification identifying component configured to: identify a query state modification set comprising at least two candidate query state modifications respectively having a query state modification probability, and from the query state modification set, identify a selected query state modification having a higher query state modification probability for the follow-up query than the query state modifications of other candidate query state modifications; and a query state modifying component configured to modify the query state according to the selected query state modification and the query terms identified in the follow-up query. 10. The system of claim 9 , wherein: the system further comprises a query state modification training set; the query state modification identifying component comprises a classifier; the query state modification identifying component is further configured to train the classifier using the query state modification training set; and selecting the query state modification set comprises: after training the classifier, classifying respective queries into a query state modification selected from the query state modification set.