anchor.diff:  ;;
   ;;; And [it can't be said too often] unicode is not a glyph encoding
   ;;; but more a kind of text formating.
  -;;; 
  +;;;
   ;;; [1] or even a code position
   ;;; --GB
   
  
positive.code: 
  (defmethod (setf medium-clipping-region) :after (region (medium medium))
    (declare (ignore region))
    (let ((sheet (medium-sheet medium)))    
      (when sheet
        (%invalidate-cached-device-regions sheet))))
  
  
negative.code:         :accessor medium-ink)
     (transformation :type transformation
                     :initarg :transformation
                     :initform +identity-transformation+
                     :accessor medium-transformation)
     (clipping-region :type region
                      :initarg :clipping-region
  