Fomus

(fomus
 :output '(:lilypond :view t)
 :parts (list
     (make-part
      :name "Percussion"
      :instr (list :percussion :percs (list (make-perc :woodblock :voice 1 :note 'e4)
                        (make-perc :snaredrum :voice 2 :note 'a3)))
      :events (loop for o from 0 to 20 by 1/2 collect
            (make-note :off o :dur 1/2
                   :note (case (random 2)
                       (0 :woodblock)
                       (1 :snaredrum)))))))