anchor.diff:                         :initform (make-text-style :fix :roman :normal)
   		       :accessor medium-default-text-style)
      (sheet :initarg :sheet
  +	  :initform nil ; this means that medium is not linked to a sheet
   	  :accessor medium-sheet)
      ))
   
  
positive.code: 
  (defmethod (setf medium-transformation) :after (transformation (medium medium))
    (declare (ignore transformation))
    (medium-invalidate-cached-device-transformation (medium-sheet medium)))
  
  (defmethod medium-device-transformation :before ((medium medium))
    (with-slots (device-transformation) medium
  
negative.code: 
  (defmethod (setf medium-clipping-region) :after (clipping-region (medium medium))
    (declare (ignore clipping-region))
    (when (medium-sheet medium)
      (medium-invalidate-cached-device-region (medium-sheet medium))))
  
  (defmethod medium-device-region :before ((medium medium))
    (with-slots (device-region) medium
  