anchor.diff:  (defpackage #:clim-tab-layout
     (:use #:clim #:clim-lisp)
     (:export #:tab-layout
  -	   #:tab-layout-pane
  -	   #:tab-layout-pages
  -	   #:tab-page
  -	   #:tab-page-tab-layout
  -	   #:tab-page-title
  -	   #:tab-page-pane
  -	   #:tab-page-presentation-type
  -	   #:tab-page-drawing-options
  -	   #:add-page
  -	   #:remove-page
  -	   #:tab-layout-enabled-page
  -	   #:sheet-to-page
  -	   #:find-tab-page-named
  -	   #:switch-to-page
  -	   #:remove-page-named
  -	   #:with-tab-layout
  +           #:tab-layout-pane
  +           #:tab-layout-pages
  +           #:tab-page
  +           #:tab-page-tab-layout
  +           #:tab-page-title
  +           #:tab-page-pane
  +           #:tab-page-presentation-type
  +           #:tab-page-drawing-options
  +           #:add-page
  +           #:remove-page
  +           #:tab-layout-enabled-page
  +           #:sheet-to-page
  +           #:find-tab-page-named
  +           #:switch-to-page
  +           #:remove-page-named
  +           #:with-tab-layout
              #:com-switch-to-tab-page
  -	   #:com-remove-tab-page
  -	   #:note-tab-page-changed))
  +           #:com-remove-tab-page
  +           #:note-tab-page-changed))
   
   (in-package #:clim-tab-layout)
   
  
positive.code:     (dolist (page pages)
        (setf (sheet-enabled-p (tab-page-pane page)) (eq page current))))
    (let ((header
  	 (make-pane 'tab-bar-pane
  	  :display-time :command-loop
  	  :display-function
  	  (lambda (frame pane)
  	    (declare (ignore frame))
  	    (funcall (header-display-function instance) instance pane)))))
      (setf (tab-layout-header-pane instance) header)
      (sheet-adopt-child instance header)
      (setf (sheet-enabled-p header) t)))
  
negative.code:   ;; do the call only after the change has been done:
    (let ((old-page (tab-layout-enabled-page parent)))
      (prog1
          (call-next-method)
        (when (and page (not (equal page old-page)))
          (note-tab-page-enabled page)))))
  
  (defmethod (setf tab-layout-pages) (newval (parent tab-layout))
    (unless (equal newval (remove-duplicates newval))
      (error "page list must not contain duplicates: ~A" newval))
    (let* ((oldval (tab-layout-pages parent))
           (add (set-difference newval oldval))
           (remove (set-difference oldval newval)))
      ;; check for errors
      (dolist (page add)
        (unless (null (tab-page-tab-layout page))
          (error "~A has already been added to a different tab layout" page)))
      ;; remove old pages first, because sheet-disown-child still needs access
      ;; to the original page list:
      (dolist (page remove)
  