12. A system for evaluating a dialogue with a user on a device having a processor and a memory, the system comprising: a dialogue hypothesis set; an expression evaluator comprising instructions stored in the memory that, when executed on the processor, cause the device to, for respective expressions of the dialogue: apply an expression recognizer and a natural language processor to: upon receiving from the user an expression replacing a previous subject of the dialogue with a substitute subject, store in the dialogue hypothesis set at least one dialogue hypothesis of the expression, the at least one dialogue hypothesis respectively comprising: at least one slot associated with the substitute subject of the expression, and a hypothesis probability; and for respective previous dialogue hypotheses in the dialogue hypothesis set that were generated for a previous expression of the dialogue, update the previous subject of at least one previous dialogue hypothesis with the substitute subject; and in response to updating the previous subject of the at least one slot, adjust the hypothesis probabilities of the respective dialogue hypotheses using a knowledge source; and a dialogue hypothesis comparator comprising instructions stored in the memory that, when executed on the processor, cause the device to: rank the dialogue hypothesis set according to the hypothesis probabilities; discard a low-ranking dialogue hypothesis from the dialogue hypothesis set; adjust the hypothesis probabilities of the dialogue hypothesis set; and for a high-ranking dialogue hypothesis having a hypothesis probability exceeding a hypothesis confidence threshold, execute an action fulfilling the high-ranking dialogue hypothesis, wherein executing the action comprises presenting to the user information associated with the high-ranking dialogue hypothesis.
14. The system of claim 12 , wherein: the previous subject further comprises a subject genus within a current knowledge domain of the knowledge source; and the substitute subject further comprises a subject species within the subject genus within the current knowledge domain.