Documentation: simple-draw.ss

   start            ; Number Number -> true
   ;; (start width height)
   ;; create a canvas with the given dimension (width x height)

   big-bang         ; Number X -> true
   ;; (big-bang n x)				 
   ;; start the clock ticking every n seconds and set the world to x

   on-tick-event    ; (X -> X) -> true
   ;; (on-tick-event tock)
   ;; set the tick handler to tock, which is used for every tick of the clock
   ;; tock consumes the current world and produces a new one 

   end-of-time      ; -> X 
   ;; (end-of-time)			
   ;; stop the clock and produce the current world

   on-key-event     ; (KeyEvent X -> X) -> true
   ;; (on-key-event click)
   ;; set the key event handler to click, which is used for every click of
   ;; a keyboard key
   ;; click consumes a KeyEvent (a Char or a Symbol) and the current world,
   ;; and produces a new world 

   draw-solid-disk0 ; Number Number Number Color -> true
   ;; (draw-solid-disk0 x y r c)
   ;; draw a colored disk at (x,y) with radius r and color c

   draw-solid-rect0 ; Number Number Number Number Color -> true
   ;; (draw-solid-rect0 x y w h c)
   ;; draw a colored rectangle at (x,y) of width w, height h, and color c

   draw-solid-line0 ; Number Number Number Number Color -> true
   ;; (draw-solid-line0 x0 y0 x1 y1 c)
   ;; draw a colored line from (x0,y0) to (x1,y1) of color c