anchor.diff:    (let ((ptypevar (gensym)))
       `(let ((,ptypevar ,default-presentation-type))
          (make-pane 'tab-layout
  -		  :name ,(or name `',(gensym "tab-layout-"))
  -		  :pages (list ,@(mapcar (lambda (spec)
  -					   `(make-tab-page ,@spec
  -							   :presentation-type
  -					     ,ptypevar))
  -					 body))
  -		  ,@initargs))))
  +                  :name ,(or name `',(gensym "tab-layout-"))
  +                  :pages (list ,@(mapcar (lambda (spec)
  +                                           `(make-tab-page ,@spec
  +                                                           :presentation-type
  +                                             ,ptypevar))
  +                                         body))
  +                  ,@initargs))))
   
   (defun make-tab-page
       (title pane &key presentation-type drawing-options enabled-callback)
  
positive.code:    (title :initform nil :accessor tab-page-title :initarg :title)
     (pane :initform nil :accessor tab-page-pane :initarg :pane)
     (presentation-type :initform 'tab-page
  		      :accessor tab-page-presentation-type
  		      :initarg :presentation-type)
     (enabled-callback :initform nil
  		     :accessor tab-page-enabled-callback
  		     :initarg :enabled-callback)
     ;; fixme: drawing-options in this generality are a feature of the old
     ;; concrete tab pane.  Gtkairo will only look for the :INK in this list.
     (drawing-options :initform nil
  		    :accessor tab-page-drawing-options
  		    :initarg :drawing-options))
    (:documentation "Instances of TAB-PAGE represent the pages in a TAB-LAYOUT.
  For each child pane, there is a TAB-PAGE providing the page's title and
  additional information about the child.  Valid initialization arguments
  
negative.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)))
  