(define (new-instance class . parameters)
 (let ((instance (apply class parameters)))
   (virtual-operations instance)
   instance))

(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)))))

; 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 (object)
  (let ((super '())
        (self 'nil))

   (define (set-self! object-part)
     (set! self object-part))

   (define (dispatch message)
     (cond ((eqv? message 'set-self!) set-self!)
           (else '())))
 
   (set! self dispatch)
   self))


(define (point x y)
 (let ((super (new-part object))
       (self 'nil))
  (let ((x x) 
        (y y)
       )
     
     (define (getx) x)
  
     (define (gety) y)
  
     (define (add p) 
      (point 
       (+ x (send 'getx p))
       (+ y (send 'gety p))))
  
     (define (type-of) 'point)

     (define (point-info)
       (list (send 'getx self) (send 'gety self) (send 'type-of self)))

     (define (set-self! object-part)
         (set! self object-part)
         (send 'set-self! super object-part))
       
     (define (dispatch message)
       (cond ((eqv? message 'getx) getx)
             ((eqv? message 'gety) gety)
             ((eqv? message 'add)  add)
             ((eqv? message 'point-info)  point-info)
             ((eqv? message 'type-of) type-of)
             ((eqv? message 'set-self!) set-self!)
             (else (method-lookup super message))))
     
     (set! self dispatch)
   )

   self))  


(define (color-point x y color)
 (let ((super (new-part point x y))
       (self 'nil))
   (let ((color color))
       
     (define (get-color)
       color)

     (define (type-of) 'color-point)

     (define (set-self! object-part)
         (set! self object-part)
         (send 'set-self! super object-part))
       
     (define (dispatch message)
       (cond ((eqv? message 'get-color) get-color)
             ((eqv? message 'type-of) type-of)
             ((eqv? message 'set-self!) set-self!)
             (else (method-lookup super message))))
       
     (set! self dispatch)
   )
     
   self))