Exercise 6.2.4:

```;; dimensions of traffic light
(define WIDTH 50)
(define HEIGHT 160)
(define BULB-DISTANCE 10)

;; the positions of the bulb
(define X-BULBS (quotient WIDTH 2))
(define Y-YELLOW (+ Y-RED BULB-DISTANCE (* 2 BULB-RADIUS)))
(define Y-GREEN (+ Y-YELLOW BULB-DISTANCE (* 2 BULB-RADIUS)))

;; clear-bulb: symbol -> symbol
(define (clear-bulb bulb)
(cond
[(symbol=? bulb 'red) (and (hide-disk bulb)
(show-circle bulb))]
[(symbol=? bulb 'yellow) (and (hide-disk bulb)
(show-circle bulb))]
[(symbol=? bulb 'green) (and (hide-disk bulb)
(show-circle bulb))]))

;; draw-bulb: symbol -> symbol
(define (draw-bulb bulb)
(cond
[(symbol=? bulb 'red) (and (hide-circle bulb)
(show-disk bulb))]
[(symbol=? bulb 'yellow) (and (hide-circle bulb)
(show-disk bulb))]
[(symbol=? bulb 'green) (and (hide-circle bulb)
(show-disk bulb))]))

;; switch: symbol symbol -> symbol
(define (switch bulb1 bulb2)
(and (clear-bulb bulb1)
(draw-bulb bulb2)))

;; hide-disk: symbol -> symbol
(define (hide-disk bulb)
(cond
[(symbol=? bulb 'red)
(clear-solid-disk (make-posn X-BULBS Y-RED) BULB-RADIUS 'red)]
[(symbol=? bulb 'yellow)
(clear-solid-disk (make-posn X-BULBS Y-YELLOW) BULB-RADIUS 'yellow)]
[(symbol=? bulb 'green)
(clear-solid-disk (make-posn X-BULBS Y-GREEN) BULB-RADIUS 'green)]))

;; show-circle: symbol -> symbol
(define (show-circle bulb)
(cond
[(symbol=? bulb 'red)
(draw-circle (make-posn X-BULBS Y-RED) BULB-RADIUS 'red)]
[(symbol=? bulb 'yellow)
(draw-circle (make-posn X-BULBS Y-YELLOW) BULB-RADIUS 'yellow)]
[(symbol=? bulb 'green)
(draw-circle (make-posn X-BULBS Y-GREEN) BULB-RADIUS 'green)]))

;; hide-cicle: symbol -> symbol
(define (hide-circle bulb)
(cond
[(symbol=? bulb 'red)
(clear-circle (make-posn X-BULBS Y-RED) BULB-RADIUS 'red)]
[(symbol=? bulb 'yellow)
(clear-circle (make-posn X-BULBS Y-YELLOW) BULB-RADIUS 'yellow)]
[(symbol=? bulb 'green)
(clear-circle (make-posn X-BULBS Y-GREEN) BULB-RADIUS 'green)]))

;; show-circle: symbol -> symbol
(define (show-disk bulb)
(cond
[(symbol=? bulb 'red)
(draw-solid-disk (make-posn X-BULBS Y-RED) BULB-RADIUS 'red)]
[(symbol=? bulb 'yellow)
(draw-solid-disk (make-posn X-BULBS Y-YELLOW) BULB-RADIUS 'yellow)]
[(symbol=? bulb 'green)
(draw-solid-disk (make-posn X-BULBS Y-GREEN) BULB-RADIUS 'green)]))

;; draw the light with the red bulb turned on
(start WIDTH HEIGHT)
(draw-solid-disk (make-posn X-BULBS Y-RED) BULB-RADIUS 'red)
(draw-circle (make-posn X-BULBS Y-YELLOW) BULB-RADIUS 'yellow)
(draw-circle (make-posn X-BULBS Y-GREEN) BULB-RADIUS 'green)
```