anchor.diff:        (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 add)))))
  +      (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))
  
positive.code:     (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)))))
  
  (defun sheet-to-page (sheet)
    "For a SHEET that is a child of a tab layout, return the page corresponding
  
negative.code: 
  (defmethod note-tab-page-changed ((layout tab-layout) page)
    nil)
  
  (defun note-tab-page-enabled (page)
    (let ((callback (tab-page-enabled-callback page)))
  