(cond
  ((or (not (defined? 'canvas)) (not (image? canvas)))
   (define canvas (image.new 200 200))
   (image.show canvas)))

(define fractal-rectangle!
  (lambda (image color left top right bottom level)
    (cond
      ((= level 0)
       (envt.set-fgcolor! color)
       (image.select-rectangle! image selection.replace
                                left top 
                                (- right left)
                                (- bottom top))
       (image.fill! image)
       (image.select-nothing! image)
       (envt.update-displays!))
      (else
       (let* ((midcol1 (round (+ left (/ (- right left) 3))))
              (midcol2 (round (- right (/ (- right left) 3))))
              (midrow1 (round (+ top (/ (- bottom top) 3))))
              (midrow2 (round (- bottom (/ (- bottom top) 3)))))
         ; First row of squares
         (fractal-rectangle! image 
                             color
                             left top 
                             midcol1 midrow1
                             (- level 1))
         (fractal-rectangle! image 
                             (rgb.complement color)
                             midcol1 top 
                             midcol2 midrow1
                             (- level 1))
         (fractal-rectangle! image 
                             color
                             midcol2 top 
                             right midrow1
                             (- level 1))
         ; Second row of squares
         (fractal-rectangle! image 
                             (rgb.complement color)
                             left midrow1
                             midcol1 midrow2
                             (- level 1))
         (fractal-rectangle! image 
                             color
                             midcol1 midrow1
                             midcol2 midrow2
                             (- level 1))
         (fractal-rectangle! image 
                             (rgb.complement color)
                             midcol2 midrow1
                             right midrow2
                             (- level 1))
         ; Third row of squares
         (fractal-rectangle! image 
                             color
                             left midrow2
                             midcol1 bottom
                             (- level 1))
         (fractal-rectangle! image 
                             (rgb.complement color)
                             midcol1 midrow2
                             midcol2 bottom
                             (- level 1))
         (fractal-rectangle! image 
                             color
                             midcol2 midrow2
                             right bottom
                             (- level 1))
         )))))