Representing Drums With Grammar

I'm still trying to resolve the grammatical method to work with drums and other percussive instruments. The problem is that, while melodic instruments only require notes (and details such as velocity, panning, etc.), drums require notes that correspond to different drums. Thus, the notes are not related in the same way that melodic notes are related in. Drums have no "key" and do not play contoured riffs.

Representing drum patterns within a grammar will require a more flexible grammar that allows for multiple notes on the same beat. I'm thinking of using a very low-level grammar to describe individual hit patterns on the different drums, then combining these in higher-level grammars that describe riffs, fills, and grooves that finally combine into the highest-level grammar that describes an entire style of playing.