anchor.diff:  	and x = 0 then (+ x nw)
   	and nh = (/ (* height (get-pane-height child)) (get-pane-height box))
           do (move-sheet child x 0)
  -	   (setf nh (do-in nh (sr-min-height child) (sr-max-height child)))
  +	   (setf nh (clamp nh (sr-min-height child) (sr-max-height child)))
   	   (allocate-space child nw (round nh))))
   
   ;;; VBOX-PANE
  
positive.code: 	  and y = 0 then (+ y nh)
  	  and nw = (/ (* width (get-pane-width child)) (get-pane-width box))
  	  do (move-sheet child 0 y)
  	     (setf nw (do-in nw (sr-min-width child) (sr-max-width child)))
  	     (allocate-space child (round nw) nh)))
    
  ;;; HRACK-PANE
  
negative.code:   (:documentation ""))
  
  (defmethod compose-space ((rack vrack-pane))
    (loop for child in (sheet-children rack)
          for sr = (compose-space child)
          maximize (space-requirement-width sr) into width
          maximize (space-requirement-min-width sr) into min-width
          minimize (space-requirement-max-width sr) into max-width
          sum (space-requirement-height sr) into height
          sum (space-requirement-min-height sr) into min-height
          sum (space-requirement-max-height sr) into max-height
          finally
          (progn
            (setf (slot-value rack 'sum-height) (max min-height height))
            (return (make-space-requirement
                     :min-width  min-width
                     :width      (max min-width width)
                     :max-width  (max max-height width)
                     :min-height min-height
                     :height     (max min-height height)
                     :max-height (max max-height height))) )))
  
  (defmethod compute-extremum ((rack vrack-pane))
    (make-extremums-children-ratio-height rack (sheet-children rack)))
  