9. The non-transitory computer storage medium of claim 8 , wherein for each action, adjusting the corresponding set of command sentences for the action based on the first n-grams and the second n-grams comprises: determining a first proper subset of the first n-grams that are determined to be most semantically relevant; determining a second proper subset of the second n-grams that are determined to be most semantically irrelevant; and selecting candidate command sentences that include at least one first n-gram from the first proper subset and at least one second n-gram from the second proper subset.
10. The non-transitory computer storage medium of claim 9 , wherein selecting candidate command sentences that include at least one first n-gram from the first proper subset and at least one second n-gram from the second proper subset comprises: scoring each candidate command sentence for semantic relevance to the action; and selecting only the candidate command sentences having scores that meet a threshold indicating a minimum semantic relevance.