1. A computer-implemented method, comprising: accessing command sentences stored in a data store, wherein each command sentence is a set of n-grams that constitute the command sentence and each command sentence includes a plurality of n-grams, wherein the command sentences include n-grams that collectively map to a plurality of n-gram types; for each of the n-gram types: identifying n-gram spans, each n-gram span being a proper subset of a set of n-grams that constitute a command sentence and including first n-grams of the n-gram type and one or more second n-grams that do not map to any of the plurality of n-gram types; determining clusters of the n-gram spans, each cluster including n-gram spans meeting a measure of similarity of n-grams spans that belong to the cluster; and for each cluster of n-gram spans, determining, from the n-gram spans belonging to the cluster, a new type to which the n-grams of the n-gram spans map.