anchor.diff:    (dolist (child (alexandria:ensure-list contents))
       (sheet-adopt-child sheet child)))
   
  -(defmethod compose-space ((bboard bboard-pane) &key width height)
  -  (declare (ignore width height))
  -  (make-space-requirement :width 300 :height 300))
  +(defmethod compose-space ((bboard bboard-pane) &key (width 100) (height 100))
  +  (make-space-requirement :width width :height height))
   
   ;;; VIEWPORT
   
  
positive.code:    (user-min-height :accessor %pane-user-min-height)
     (user-max-height :accessor %pane-user-max-height)
     ;; size required by the stream
     (stream-width :initform 0 :accessor stream-width)
     (stream-height :initform 0 :accessor stream-height))
    (:documentation
     "This class implements a pane that supports the CLIM graphics,
      extended input and output, and output recording protocols."))
  
negative.code: (defmethod (setf medium-background) (ink (pane pane))
    (setf (medium-background (sheet-medium pane)) ink))
  
  (defmethod compose-space ((pane pane) &key (width 100) (height 100))
    (make-space-requirement :width width :height height))
  
  (defmethod allocate-space ((pane pane) width height)
    (declare (ignorable pane width height))
  