;constants
(define background (empty-scene 450 150))
(define red (place-image
             (overlay/offset
              (overlay/offset (overlay
                               (circle 65 255 "white")
                               (circle 70 255 "green"))
                                         -145 0
                                         (overlay
                                          (circle 65 255 "white")
                                          (circle 70 255 "yellow")))
              -217 0 (circle 70 255 "red")) 225 75 background))
(define yellow (place-image
             (overlay/offset
              (overlay/offset (overlay
                               (circle 65 255 "white")
                               (circle 70 255 "green"))
                                         -145 0
                                    (circle 70 255 "yellow"))
                       -217 0 (overlay (circle 65 255 "white")
                         (circle 70 255 "red"))) 225 75 background))
(define green (place-image
             (overlay/offset
              (overlay/offset
                                   (circle 70 255 "green")
                                         -145 0
                                   (overlay (circle 65 255 "white")
                                   (circle 70 255 "yellow")))
                       -217 0 (overlay (circle 65 255 "white")
                         (circle 70 255 "red"))) 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")