; constants
(define background (empty-scene 150 150))
(define red (place-image (circle 70 255 "red") 75 75 background))
(define yellow (place-image (circle 70 255 "yellow") 75 75 background))
(define green (place-image (circle 70 255 "green") 75 75 background))
(define (FunctionForDraw x)
  (cond
    [(string=? "green" x) red]
    [(string=? "red" x) yellow]
    [(string=? "yellow" x) green]))
(define (FunctionForTick x)
  (+ 1 x))
; functions
(define svetofor
 (big-bang "green"
  (to-draw FunctionForDraw)
  (on-tick FunctionForTick 2)))