anchor.diff:  
   (defmethod pane-viewport ((pane basic-pane))
     (let ((parent (sheet-parent pane)))
  -    (if (and parent (typep parent 'viewport-pane))
  -	parent
  -      nil)))
  +    (when (and parent (typep parent 'viewport-pane))
  +	parent)))
   
   ;;; Default for streams that aren't even panes.
   
  
positive.code: 
  (defmethod pane-scroller ((pane basic-pane))
    (let ((viewport (pane-viewport pane)))
      (if viewport
  	(sheet-parent viewport))))
  
  (defmethod scroll-extent ((pane basic-pane) x y)
  
negative.code: 
  (defmacro bordering ((&rest options) &body contents)
    `(make-pane 'border-pane ,@options :contents (list ,@contents)))
  
  (defmethod pane-border ((pane basic-pane))
    (let ((parent (sheet-parent pane)))
      (when (and parent (typep parent 'border-pane))
  	parent)))
  
  ;;; RAISED PANE
  
  