anchor.diff:  
   (defmethod sheet-disown-child :before ((parent tab-layout) child &key errorp)
     (declare (ignore errorp))
  -  (alexandria:when-let ((page (sheet-to-page child)))
  +  (when-let ((page (sheet-to-page child)))
       (setf (slot-value parent 'pages) (remove page (tab-layout-pages parent))
             (tab-page-tab-layout page) nil)
       (when (eq page (tab-layout-enabled-page parent))
  
positive.code: 
  (defpackage #:clim-tab-layout
    (:use #:clim #:clim-lisp)
    (:export #:tab-layout
             #:tab-layout-pane
             #:tab-layout-pages
  
negative.code: 
  (defmethod (setf tab-page-title) :after (newval (page tab-page))
    (declare (ignore newval))
    (when-let ((layout (tab-page-tab-layout page)))
      (note-tab-page-changed layout page)))
  
  (defmethod (setf tab-page-drawing-options) :after (newval (page tab-page))
    (declare (ignore newval))
    (when-let ((layout (tab-page-tab-layout page)))
      (note-tab-page-changed layout page)))
  
  (defmethod note-tab-page-changed ((layout tab-layout) page)
    nil)
  
  (defun note-tab-page-enabled (page)
    (when-let ((callback (tab-page-enabled-callback page)))
      (funcall callback page)))
  
  
  ;;; convenience functions:
  