(define chords (list (list 60 61 62) (list 61 63 65) (list 63 66 69) (list 66 70 74)))

(define (make-chord start duration amplitude pitches)
  (if (null? pitches)
    nil
    (cons (new midi
            :time start
            :keynum (car pitches)
            :duration duration
            :amplitude amplitude)
          (make-chord start duration amplitude (cdr pitches)))))

(define (make-chord-sequence start duration amplitude chords)
  (if (null? chords)
    nil
    (append (make-chord start duration amplitude (car chords))
            (make-chord-sequence (+ start duration) duration amplitude (cdr chords)))))