オブジェクトシステム(2)

メソッド
  (define-method write-object ((self <2d-point>) port)
    (format port "#<2d-point ~a ~a>"
                 (ref self 'x) (ref self 'y)))

  pt   ⇒   #<2d-point 30 20>

  (define-method move! ((self <2d-point>) 
                        (dx <real>) (dy <real>))
    (inc! (ref self 'x) dx)
    (inc! (ref self 'y) dy))

  (move! pt -3 5)
  pt   ⇒   #<2d-point 27 25>