1. A machine comprising: a) a non-transitory computer readable medium storing a plurality of grammars, wherein: i) the plurality of grammars is organized according to a hierarchy comprising a plurality of levels; ii) for each level above the lowest level in the hierarchy, each grammar from that level comprises one or more elements provided by a grammar from the next lower level; iii) a grammar from the lowest level in the hierarchy comprises a vocabulary comprising words corresponding to natural language statements; b) a computer configured with a set of computer executable instructions to perform a set of acts comprising: i) receiving a natural language input from a user; and ii) using at least two grammars from the plurality of grammars, determining a prompt to provide in response to the natural language input from the user.