[racket] simple question from newb

Wrote this from the lesson:

;; this is the definitive exercise 32 from the 2htdp
;; car moves across the empty-screen.  Yay.  12/2/2010


;;  this creates the empty window

(define WIDTH 1000)
(define HEIGHT 40)
(define MTSCN (empty-scene WIDTH HEIGHT))

;; this draws the vehicle shape

(define WHEEL-RADIUS 5)
  (define WHEEL-DISTANCE (* WHEEL-RADIUS 5))
  (define BODY-LENGTH (+ WHEEL-DISTANCE (* 6 WHEEL-RADIUS)))
  (define BODY-HEIGHT (* WHEEL-RADIUS 2))
   (define WHL (circle WHEEL-RADIUS "solid" "black"))
  (define BDY
    (above
      (rectangle (/ BODY-LENGTH 2) (/ BODY-HEIGHT 2)
                 "solid" "blue")
      (rectangle BODY-LENGTH BODY-HEIGHT "solid" "red")))
  (define SPC (rectangle WHEEL-DISTANCE 1 "solid" "white"))
  (define WH* (beside WHL SPC WHL))
  (define CAR (underlay/xy BDY WHEEL-RADIUS BODY-HEIGHT WH*))

  ;; As Ramin A. my old classmate once said, "I drew a tree"
  ;; Well, this is a tree I was instructed to place in the empty screen
  ;; as a constant shape throughout the running fo the program

  (define tree
    (underlay/xy (circle 10 'solid 'green)
                 9 15
                 (rectangle 2 20 'solid 'brown)))
;; this one actually places the tree!

   (define BACKGROUND
     (place-image tree 500 25 MTSCN)
     )

;; this sets the y-axis (vertical) for the car and
;; defines the "tock" where the trigger is the clock (not keystroke or
mouse)
;; increments of 3

  (define Y-CAR 30)
  (define (tock ws) (+ ws 3))

;;  Here we are placing the car into the scene, according to the given world
state

  (define (render ws) (place-image CAR ws Y-CAR BACKGROUND))



;;  here is the big-bang, or main program, that is the actual instruction
;; to use all the above-definitions

  (define (main ws)
    (big-bang ws (on-tick tock) (to-draw render) *(stop-when (> ws 500)  )*
))

;;  that's it!
;;  next step is a condition that will stop the program when
;; the vehicle reaches a certain point

****************************
Ok, I bold-faced the attempt to have the car stop at x-axis of 500, which I
believe is ws 500.  When I put in "(main 1)" in the execution line, I get a
false boolean, if I put in "(main 501)" I get a true boolean.  Both cases,
it doesn't work the "procedure expects an argument", not a true/false.

How the heck can I get the stop-when function to work, or can I be pointed
to a section of the tutor or other reference that explains to me what should
be a simple (not to me) additional command in the big-bang.  Thank you in
advance,

dt

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