(define point%
  (class object%
    (super-new)

    (init-field [x 0] [y 0])

    (define/public (move dx dy)
      (new point% [x (+ x dx)] [y (+ y dy)]))

    (define/public (move! dx dy)
      (set! x (+ x dx))
      (set! y (+ y dy)))

    ))

(define color-point%
  (class point%
    (super-new)
    (inherit-field x y)

    (init-field [color 'black])

    (define/override (move dx dy)
      (new color-point% [x (+ x dx)] [y (+ y dy)]))

    (define/public (paint c)
      (new color-point% [x x] [y y] [color c]))

    (define/public (paint! c)
      (set! color c))

    ))

(define P (send (send (new color-point%) move 3 4) paint 'red))
(read-line)
(send* P
  (move! 2 8)
  (paint! 'blue))