anchor.diff:      (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))
  -      (setf (tab-layout-enabled-page parent) (page-successor page)))))
  +      (setf (tab-layout-enabled-page parent) (car (tab-layout-pages parent))))))
   
   (defun sheet-to-page (sheet)
     "For a SHEET that is a child of a tab layout, return the page corresponding
  
positive.code: 
  (defmethod note-tab-page-changed ((layout tab-layout) page)
    nil)
  
  (defun page-successor (page)
    "The page we should enable when PAGE is currently enabled but gets removed."
    (loop for (a b c) on (tab-layout-pages (tab-page-tab-layout page)) do
  	(cond
  	  ((eq a page) (return b))
  	  ((eq b page) (return (or c a))))))
  
  (defun note-tab-page-enabled (page)
    (let ((callback (tab-page-enabled-callback page)))
  
negative.code:       (sheet-adopt-child parent (tab-page-pane page)))
      ;; ensure that at least one page is enabled
      (when (null (tab-layout-enabled-page parent))
        (setf (tab-layout-enabled-page parent) (car (tab-layout-pages parent))))))
  
  (defmethod sheet-disown-child :before ((parent tab-layout) child &key errorp)
    (declare (ignore errorp))
  