music~

#lang racket
; OOP Helper methods
; Instantiate method
(define (new-instance class . parameters)
 (let ((instance (apply class parameters)))
   (virtual-operations instance)
   instance))

; Arrange for virtual operations in object
(define (virtual-operations object)
  (send 'set-self! object object))

; 
(define (new-part class . parameters)
  (apply class parameters))

;
(define (method-lookup object selector)
  (cond ((procedure? object) (object selector))
         (else (error "Inappropriate object in method-lookup: " object))))

(define (send message object . args)
  (let ((method (method-lookup object message)))
    (cond ((procedure? method) (apply method args))
          ((null? method) (error "Message not understood: " message))
          (else (error "Inappropriate result of method lookup: " method)))))

; Classes
(define (element length start-time)
  (let