(define WINDOW-WIDTH 150)
(define WINDOW-HEIGHT 180)

; A world is a string.
; Its display is a green 18-point image of its text.
; show-world : world -> image
(define (show-world world)
  (text world 18 "green"))
"Examples of show-world:"
(show-world "hello") "should be" (text "hello" 18 "green")

; The next world is formed by adding a d to the end of the previous one.
; handle-tick : world -> world
(define (handle-tick world)
  (string-append world "d"))
"Examples of handle-tick:"
(handle-tick "ace") "should be" "aced"

(big-bang WINDOW-WIDTH WINDOW-HEIGHT 1 "abc")
(on-redraw-event show-world)
(on-tick-event handle-tick)