anchor.diff:    (typep pane 'spacing-pane))
   
   (defmethod initialize-instance :after ((spacing spacing-pane) &key thickness contents &allow-other-keys)
  +  (declare (ignorable thickness contents))
     (with-slots (user-width user-min-width user-max-width
                  user-height user-min-height user-max-height)
         spacing
  
positive.code: (defclass viewport-pane (single-child-composite-pane) ())
  
  (defmethod compose-space ((pane viewport-pane) &key width height)
    ; I _think_ this is right, it certainly shouldn't be the requirements of the child.
    (make-space-requirement))
  
  
negative.code: 	(call-next-method))))
  
  (defmethod compose-space ((pane clim-stream-pane) &key width height)
    (declare (ignorable width height))
    (let ((w (bounding-rectangle-width (stream-output-history pane)))
          (h (bounding-rectangle-height (stream-output-history pane))))
      (make-space-requirement :width  w :min-width  w :max-width +fill+
  