anchor.diff:  (defclass clx-medium (basic-medium)
     ((gc :initform nil)
      (picture :initform nil)
  -   (clipping-region-dirty :initform t)
  +   (last-medium-device-region :initform nil)
      (clipping-region-tmp :initform (vector 0 0 0 0)
        :documentation "This object is reused to avoid consing in the
    most common case when configuring the clipping region.")
  
positive.code:   (let* ((port (port medium))
  	 (mirror (port-lookup-mirror port (medium-sheet medium)))
  	 (line-style (medium-line-style medium)))
      (with-slots (gc clipping-region-dirty) medium
        (unless gc
  	(setq gc (xlib:create-gcontext :drawable mirror))
  	;; this is kind of false, since the :unit should be taken
  
negative.code:             ;; to clx :] we stick with :unsorted until that can be sorted out
              (setf (xlib:gcontext-clip-mask gc :unsorted) rect-seq)))))))
  
  
  (defgeneric medium-gcontext (medium ink))
  