anchor.diff:  (defmethod sheet-native-transformation ((sheet top-level-sheet-pane))
     +identity-transformation+)
   
  -;;; SHEET
  -
  -;; FIXME: Should it exist ??? - I don't think so...
  -#+nil
  -(defmethod note-space-requirements-changed ((sheet sheet) (pane composite-pane))
  -  (values))
  -
  +(defmethod change-space-requirements ((pane unmanaged-top-level-sheet-pane)
  +                                      &rest space-req-keys
  +                                      &key resize-frame &allow-other-keys)
  +  ;; Special variant for unmanaged-top-level-sheet-pane. Since the
  +  ;; pane is unmanaged there is no window manager which can offer the
  +  ;; user options to resize this top level pane.
  +  ;;
  +  ;; This should however be changed by turning on the :resize-frame
  +  ;; option of the frame of the unmanaged-top-level-sheet-pane and
  +  ;; handle it in the method on top-level-sheet.
  +  ;;
  +  ;; This is currently not done, since:
  +  ;; . we obviously lack the :resize-frame option
  +  ;; . of some reason the frame of e.g. a command-menu is the
  +  ;;   application-frame. I am not sure if this is totally right.
  +  ;;
  +  ;; --GB 2003-03-16
  +  (let ((w (space-requirement-width (compose-space pane)))
  +        (h (space-requirement-height (compose-space pane))))
  +    (resize-sheet pane w h)
  +    (allocate-space pane w h) ))
   
   ;;;; box-layout-mixin
   
  
positive.code:            (:line  (* (car x)
                        (stream-line-height pane)))))))
  
  (defvar *changing-space-requirements* nil
    "Bound to non-NIL while within the execution of CHANGING-SPACE-REQUIREMENTS.")
  
  (defvar *changed-space-requirements* nil
    "A list of ?? whose space requirements have changed during the current
     execution of CHANGING-SPACE-REQUIREMENTS.")
  
  (defmacro changing-space-requirements ((&key resize-frame) &body body)
    `(invoke-while-changing-space-requirements (lambda () ,@body) :resize-frame ,resize-frame))
  
  (defun invoke-while-changing-space-requirements (continuation &key resize-frame)
    (let ((*changed-space-requirements* nil))
      (let ((*changing-space-requirements* t))
        (funcall continuation))
      ;; now investiage the list of changed space-requirements ...
      ))
  
  #+nil ; I don't know what this is supposed to achieve
  (defmethod change-space-requirements ((pane composite-pane)
  				      &key resize-frame
  				      (width nil width-p)
  				      (min-width nil min-width-p)
  				      (max-width nil max-width-p)
  				      (height nil height-p)
  				      (min-height nil min-height-p)
  				      (max-height nil max-height-p))
    (declare (ignore width width-p min-width min-width-p max-width max-width-p
  		   height height-p min-height min-height-p max-height max-height-p))
    (if resize-frame
        ;; we didn't find the :resize-frame option in define-application-frame
        (layout-frame (pane-frame pane))
        (note-space-requirements-changed (sheet-parent pane) pane)))
  
  ;;; SINGLE-CHILD-COMPOSITE PANE
  
  (defclass single-child-composite-pane (sheet-single-child-mixin basic-pane) ())
  
negative.code: 
  (defclass layout-protocol-mixin ()
    ((space-requirement :accessor pane-space-requirement
                        :initform nil
                        :documentation "The cache of the space requirements of the pane. NIL means: need to recompute.")
     (current-width     :accessor pane-current-width
                        :initform nil)
     (current-height    :accessor pane-current-height
  