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: 	(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) 
    ((%trans :initarg :transformation
  	   :reader transformation
  	   :initform +identity-transformation+)))
  
  (defgeneric close-path (path))
  
  
negative.code: 	    max-y computed-max-y))))
  
  ;;; A path defined as a sequence of Bezier curve segments.
  (defclass bezier-curve (path bezier-design segments-mixin bounding-rectangle-mixin) ())
  
  (defun make-bezier-thing (class point-seq)
    (assert (= (mod (length point-seq) 3) 1))
  