Fomus

(fomus
 :output '(:lilypond :view t)
 :parts (list
         (make-part
          :name "Percussion"
          :instr (list :percussion :percs (list (make-perc :woodblock :note 'e4)
                                                (make-perc :snaredrum :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)))))))