(define Point%
  (class object%
    (super-new)
    (init-field x y)
    
    (public translate)
    
    (define (translate dx dy)
      (new Point% [x (+ x dx)] [y (+ y dy)]))))

(define Polygon%
  (class object%
    (super-new)
    
    (define vertices (vector))
    
    (public add-vertex translate)
    
    (define (add-vertex v)
      (set! vertices
            (list->vector (append (vector->list vertices)
                                  (list v)))))
    
    (define (translate dx dy)
      (let* ([vs vertices]
             [size (vector-length vs)])
        (let loop ([i 0])
          (when (< i size)
            (vector-set! vs i (send (vector-ref vs i) translate dx dy))
            (loop (+ i 1))))))))

(send* (new Polygon%)
  (add-vertex (new Point% [x 0] [y 0]))
  (add-vertex (new Point% [x 5] [y 0]))
  (add-vertex (new Point% [x 0] [y 5]))
  (translate 5 5))