anchor.diff:  (defmethod compose-space ((box hbox-pane))
     (let ((space (make-space-requirement)))
       (loop for child in (sheet-children box)
  -	for request = (compose-space child)
  +	for request = (compute-and-set-space child)
   	do (incf (space-requirement-width space)
   		 (space-requirement-width request))
   	   (incf (space-requirement-max-width space)
  
positive.code:    (enabledp :initform nil
  	     :initarg :enabledp
  	     :accessor pane-enabledp)
     (space-requirement :initarg :space-requirement
  		      :initform (make-space-requirement :width 300 :max-width 300
  							:height 300 :max-height 300)
  		      :accessor pane-space-requirement)
     )
    )
  
negative.code: (defmacro spacing ((&rest options) &body contents)
    `(make-pane 'spacer-pane ,@options :contents (list ,@contents)))
  
  (defmethod compose-space ((spacer spacer-pane))
    (compute-and-set-space (first (sheet-children spacer)))
    (let ((space (make-space-requirement))
  	(margin-space (pane-space-requirement spacer))
  	(child-space (pane-space-requirement (first (sheet-children spacer)))))
  