15. A system comprising: at least one processor; and a memory storing instructions that, when executed by the at least one processor perform the following actions: receive an initial query from a user; generate an initial query state based on the initial query; receive a follow-up query from a user; and upon receiving the follow-up query: parse the follow-up query to identify at least zero follow-up query terms; calculate query state modification probabilities of the follow-up query for at least two candidate query state modifications in the query state modification set; modify the query state according to the follow-up query terms and the query state modification having a highest query state modification probability among the candidate query state modifications; and fulfill the query in view of the query state.
15. A system comprising: at least one processor; and a memory storing instructions that, when executed by the at least one processor perform the following actions: receive an initial query from a user; generate an initial query state based on the initial query; receive a follow-up query from a user; and upon receiving the follow-up query: parse the follow-up query to identify at least zero follow-up query terms; calculate query state modification probabilities of the follow-up query for at least two candidate query state modifications in the query state modification set; modify the query state according to the follow-up query terms and the query state modification having a highest query state modification probability among the candidate query state modifications; and fulfill the query in view of the query state. 16. The system of claim 15 , wherein the query state modification set comprises: a query state initiating modification; a query term adding state modification; a query term substituting state modification; a query term removing state modification; a query state saving modification; a query state sequence navigating modification; a query state domain changing modification; a query state intent changing modification; and a query state resetting modification.