anchor.diff:             (: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) ())
  
positive.code:    :display-time nil
     :scroll-bars nil
     :default-view +pointer-documentation-view+
     :height '(2 :line)
     :text-style (make-text-style :sans-serif :roman :normal)
     :foreground +white+
     :background +black+
  
negative.code: (defun top-level-sheet-pane-p (pane)
    (typep pane 'top-level-sheet-pane))
  
  (defmethod change-space-requirements ((pane top-level-sheet-pane)
                                        &rest space-req-keys
                                        &key resize-frame &allow-other-keys)
    (cond (*changing-space-requirements*
           ;; just record what we have
           (unless (find pane *changed-space-requirements* :key #'second)
             (push (list (pane-frame pane) pane resize-frame)
                   *changed-space-requirements*)))
          (t
           (let ((frame (pane-frame pane)))
             ;; ### we miss the :resize-frame option
             (cond (resize-frame
                    (layout-frame frame))
                   (t
                    (layout-frame frame (bounding-rectangle-width pane) (bounding-rectangle-height pane))))))))
  
  (defmethod compose-space ((pane top-level-sheet-pane) &key width height)
    (declare (ignore width height))
  