anchor.diff:  (defgeneric change-space-requirements (pane &rest rest))
   (defgeneric note-space-requirements-changed (sheet pane))
   
  -;;; PANES
  +;;; Space Requirements
   
   (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)
  +(defclass standard-space-requirement (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)
  
positive.code: (defclass pane (standard-sheet-input-mixin
  		;;temporary-medium-sheet-output-mixin
  		sheet-transformation-mixin
                  basic-sheet
                  )
    (
     #+ignore(foreground :initarg :foreground
  		       :initform +black+
  		       :reader pane-foreground)
     #+ignore(background :initarg :background
  		       :initform +white+
  		       :reader pane-background)
     (text-style :initarg :text-style :initform nil :reader pane-text-style)
     (name :initarg :name :initform "(Unnamed Pane)" :reader pane-name)
     (manager :initarg :manager)
  
negative.code: (defmethod* (setf window-viewport-position) (x y (pane clim-stream-pane))
    (scroll-extent pane x y))
  
  #+NIL
  (defun scroll-area (pane dx dy)
    (let ((transform (sheet-transformation pane)))
      ;; Region has been "scrolled" already.
  