Lecture 8 - slide 5 : 11

(define (send message obj . par)
  (let ((method (obj message)))
    (apply method par)))

(define (point x y)
  (letrec ((getx    (lambda () x))
           (gety    (lambda () y))
           (add     (lambda (p) 
                      (point 
                       (+ x (send 'getx p))
                       (+ y (send 'gety p)))))
           (type-of (lambda () 'point))
          )
    (lambda (message)
      (cond ((eq? message 'getx) getx)
            ((eq? message 'gety) gety)
            ((eq? message 'add)  add)
            ((eq? message 'type-of) type-of)
            (else (error "Message not understood"))))))