anchor.diff:  (defclass bboard-pane (composite-pane) ())
   
   (defmethod compose-space ((bboard bboard-pane))
  -  (make-space-requirement :width 300 :max-width 300 :min-width 300
  -			  :height 300 :max-height 300 :min-height 300))
  +  (make-space-requirement :width 300 :height 300))
   
   ;;; VIEWPORT
   
  
positive.code: 
  ;;; PANES
  
  (defconstant +fill+ :fill)
  
  (defclass space-requirement ()
    ((width :initform 1
  
negative.code:     (print-unreadable-object (space stream :type t :identity t)
        (format stream "width: ~S height: ~S" width height))))
  
  (defun make-space-requirement (&key (width 1) (height 1)
  				    (min-width 0) (min-height 0)
  				    (max-width +fill+) (max-height +fill+))
    (assert (<= 0 min-width width max-width) (min-width width max-width))
    (assert (<= 0 min-height height max-height) (min-height height max-height))
    (make-instance 'space-requirement
      :width width
      :max-width max-width
  