;constants
(define background (empty-scene 450 150))
(define greenf (overlay (circle 65 255 "white") ;зеленый выключен
                        (circle 70 255 "green")))
(define yellowf (overlay (circle 65 255 "white") ;желтый выключен
                         (circle 70 255 "yellow")))
(define redf (overlay (circle 65 255 "white") ;красный выключен
                      (circle 70 255 "red")))
(define red (place-image ;красный включен
             (overlay/offset
              (overlay/offset greenf -145 0 yellowf)
              -217 0 (circle 70 255 "red")) 225 75 background))
(define yellow (place-image ;желтый включен
             (overlay/offset
              (overlay/offset greenf -145 0
                                    (circle 70 255 "yellow"))
                       -217 0 redf) 225 75 background))
(define green (place-image ;зеленый включен
             (overlay/offset
              (overlay/offset (circle 70 255 "green")
                               -145 0 yellowf)
                       -217 0 redf) 225 75 background))
(define (tl-render x)
 (cond
    [(string=? "green" x) red]
    [(string=? "red" x) yellow]
    [(string=? "yellow" x) green]))
(define (tl-next x)
  (cond
    [(string=? "green" x) "red"]
    [(string=? "red" x) "yellow"]
    [(string=? "yellow" x) "green"]))
;functions
(define (traffic-light-simulation initial-state)
  (big-bang initial-state
    [to-draw tl-render]
    [on-tick tl-next 1]))
(traffic-light-simulation "green")