anchor.diff:  
   (defmethod compose-space ((pane single-child-composite-pane))
     (if (sheet-children pane)
  -      (compose-space (first (sheet-children pane)))
  +      (compute-and-set-space (first (sheet-children pane)))
       (make-space-requirement)))
   
   (defmethod allocate-space ((pane single-child-composite-pane) width height)
  
positive.code: 
  (defclass bboard-pane (composite-pane) ())
  
  (defmethod ask-space-children ((bboard bboard-pane))
    (values))
  
  (defmethod compute-space ((bboard bboard-pane))
    (pane-space-requirement bboard))
  
  
  ;; VIEWPORT
  
  
negative.code: (defmethod compose-space ((box hbox-pane))
    (let ((space (make-space-requirement)))
      (loop for child in (sheet-children box)
  	for request = (compute-and-set-space child)
  	do (incf (space-requirement-width space)
  		 (space-requirement-width request))
  	   (incf (space-requirement-max-width space)
  