Fomus

(fomus
 :output '(:lilypond :view t)
 :parts (list
         (make-part
          :name "Snare Drum"
          :instr '(:percussion :percs ((:snare-drum :note a3)))
          :events (loop for o from 0 to 40 by 1/2 when (= (random 2) 0) collect
                                                                        (make-note :off o
                                                                                   :note :snare-drum)))))