17. The system of claim 15 , 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: providing 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 to one or more raters; and selecting the candidate command sentences selected by the raters as being valid command sentences for the action.
15. The system of claim 14 , 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.