anchor.diff:  
   (defgeneric positive-negative-areas (design))
   
  -(defmethod positive-negative-areas ((design bezier-area))
  +(defmethod positive-negative-areas ((design bezier-design))
     (values (list design) '()))
   
   (defmethod positive-negative-areas ((design bezier-union))
  
positive.code:      (* (imagpart z) (point-x v))))
  
  (defclass bezier-design (climi::design)
    ((%or :accessor original-region :initform nil)))
  
  (defgeneric medium-draw-bezier-design* (stream design))
  
  
negative.code: 	(call-next-method))))
  
  ;;; an area defined as a closed path of Bezier curve segments
  (defclass bezier-area (area bezier-design segments-mixin bounding-rectangle-mixin) ())
  
  (defgeneric close-path (path))
  
  