Drum set uses a tonality 'drums' which is a predefined chromatic tonality starting from (b 2). This means that symbol a produces b 2 which is a bass drum. Normally you don't have to care about these symbols and you can just refer voices by name, but occasionally you may need to write manually symbol patterns that use directly symbols. You can now ask both note and symbol
You can use calc-drum-symbol, calc-drum-name and calc-drum-note for calculating the corresponding symbols, drum names and notes within a set.
