anchor.diff:  	 (margin-width (- (sr-width spacing) (sr-width child)))
   	 (margin-height (- (sr-height spacing) (sr-height child))))
       (setf margin-width
  -	  (do-in (round (/ (* width margin-width) (get-pane-width spacing)))
  +	  (clamp (round (/ (* width margin-width) (get-pane-width spacing)))
   		 (slot-value spacing 'margin-min-width)
   		 (slot-value spacing 'margin-max-width))
   	  margin-height
  -	  (do-in (round (/ (* height margin-height) (get-pane-height spacing)))
  +	  (clamp (round (/ (* height margin-height) (get-pane-height spacing)))
   		 (slot-value spacing 'margin-min-height)
   		 (slot-value spacing 'margin-max-height))
   	  (sheet-transformation child)
  
positive.code: 
  ;;; SCROLLER-PANE
  
  (defparameter *scrollbar-thickness* 12)
  
  (defclass scroller-pane (composite-pane)
    ((scroll-bar :type (member '(t :vertical :horizontal))
  
negative.code: 
  (defconstant +fill+ (expt 10 (floor (log most-positive-fixnum 10))))
  
  (defclass space-requirement () ())
  
  (defclass standard-space-requirement ()
    ((width :initform 1
  	  :initarg :width
  	  :reader space-requirement-width)
     (max-width :initform 1
  	      :initarg :max-width
  	      :reader space-requirement-max-width)
     (min-width :initform 1
  	      :initarg :min-width
  	      :reader space-requirement-min-width)
     (height :initform 1
  	   :initarg :height
  	   :reader space-requirement-height)
     (max-height :initform 1
  	       :initarg :max-height
  	       :reader space-requirement-max-height)
     (min-height :initform 1
  	       :initarg :min-height
  	       :reader space-requirement-min-height) ) )
  
  (defmethod print-object ((space space-requirement) stream)
    (with-slots (width height min-width max-width min-height max-height) space
      (print-unreadable-object (space stream :type t :identity nil)
        (format stream "width: ~S [~S,~S] height: ~S [~S,~S]"
                width
                min-width
                max-width
                height
                min-height
                max-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 'standard-space-requirement
      :width width
      :max-width max-width
      :min-width min-width
  