14. The system of claim 11 , wherein selecting related tokens for the first search session comprises: clustering search tokens from the second search session into token clusters; and selecting two or more of the related tokens from a same token cluster.
14. The system of claim 11 , wherein selecting related tokens for the first search session comprises: clustering search tokens from the second search session into token clusters; and selecting two or more of the related tokens from a same token cluster. 15. The system of claim 14 , wherein clustering the search tokens from the second search session comprises: identifying, for each search token in the second set of search queries, search queries from the second set of search queries that include the search token; and creating the token clusters to include only search tokens for which a threshold number of the identified search queries also included another search token in the cluster.