anchor.diff:  
   (defparameter *background-color* (list 1 1 1)) ;; white
   (defparameter *polygon-color* (list 1 0 0)) ;; red
  -(defparameter *clockwise-polygon-color* (list 1 1 1)) ;; white
  -(defparameter *counterclockwise-polygon-color* (list 0 1 0)) ;; green
  +(defparameter *clockwise-polygon-color* *background-color*) 
  +(defparameter *counterclockwise-polygon-color* *polygon-color*) 
   
   (defun set-source-color (color)
     (set-source-rgb (first color) (second color) (third color)))
  
positive.code:         (compute-score-for-buffers
           (image-surface-get-data s1)
           (image-surface-get-data s2))
        (/ count-and count-or))))
  
negative.code:   (assert (and *x-min* *x-max* *y-min* *y-max*
                 *buffer-width* *buffer-height*)))
  
  (defmethod draw-polygon-to-surface :around ((polygon src/types:clockwise-polygon))
    (let ((*polygon-color* *clockwise-polygon-color*))
      (call-next-method)))
  
  (defmethod draw-polygon-to-surface :around ((polygon src/types:counterclockwise-polygon))
    (let ((*polygon-color* *counterclockwise-polygon-color*))
      (call-next-method)))
  
  (defmethod draw-polygon-to-surface ((polygon cl-geometry:polygon)) 
    (set-source-color *polygon-color*)
    (set-line-width 1)
    (let ((points (mapcar #'draw-coordinates (cl-geometry:point-list polygon))))
  