;;; Procedure:
;;;   forbidden
;;; Parameters:
;;;   image, an image
;;;   x, the x coordinate of the center of the sign
;;;   y, the y coordinate of the center of the sign
;;;   radius, the radius of the sign
;;; Purpose:
;;;   Draws a "forbidden" sign (a circle with a slash) in
;;;   red centered at the specified location.
;;; Preconditions:
;;;   image and layer are initialized.
;;;   x, y, and radius are not negative.
;;; Postconditions:
;;;   The image now contains the specified sign.
;;;   The brush and foreground color may have changed.
(define forbidden
  (lambda (image x y radius)
    ; Choose a color and paintbrush that seem appropriate.
    (set-fgcolor RED)
    (set-brush "Circle (05)")
    ; Select the ellipse for the circle.  Do the math to see why.
    ; the particular values were set.
    (select-ellipse image REPLACE
                    (- x radius) (- y radius)
                    (* 2 radius) (* 2 radius))
    ; Draw the nice red circle.
    (stroke image)
    ; Now we're ready to draw the slash.  Some math tells us that
    ; it's offset by radius/(sqrt 2) from the center (in each
    ; direction).
    (let ((offset (/ radius (sqrt 2))))
      (line image
            (- x offset) (- y offset)
            (+ x offset) (+ y offset)))
    ; Unselect all
    (select-nothing image)))