1. A computer-implemented method performed by a data processing apparatus, the method comprising: accessing command input logs storing data defining user device sessions; identifying, from the command input logs, user device sessions that each respectively store: a sequence of two or more input sentences, each input sentence being one or more n-grams, and each input sentence having a respective ordinal position in the sequence, and wherein the sequence of two or more input sentences includes at least one pair of a first input sentence that precedes a second input sentence in ordinal position in the sequence; and data indicating a first action performed by a user device from which the user device session occurred, the first action having been performed in response to the second input sentence being successfully parsed by a parsing rule that is associated with the first action; identifying pairs of first and second input sentences from respective user device sessions that are indicative of an augmentation signal, comprising identifying pairs of first and second input sentence for which the first action is not performed in response to the first input sentence and is performed in response to the second input sentence; and determining, from the identified pairs of first and second input sentences, first input sentences for which a parsing rule that is associated with the first action is to be generated; wherein the determining first input sentences for which the parsing rule that is associated with the first action is to be generated comprises, for each first input sentence: determining a co-occurrence measure for the occurrence of first input sentence and the occurrence of the first action for the second input sentence; and determining a parsing rule that is associated with the first action is to be generated for the first input sentence when co-occurrence measure for the first input sentence and the first action meets a threshold value.