anchor.diff:      (buffer-filename buffer)))
   
   (defun push-undo-stack (buffer elt)
  -  (push elt (buffer-undo-stack buffer)))
  +  (tlist-add-right (buffer-undo-stack buffer) elt))
   
   (defun push-redo-stack (buffer elt)
     (push elt (buffer-redo-stack buffer)))
  
positive.code:     (setf (buffer-cache-linum buffer) 1)
      (setf (buffer-keep-binfo buffer) nil)
      (setf (buffer-nlines buffer) 1)
      (setf (buffer-undo-stack buffer) nil)
      (setf (buffer-redo-stack buffer) nil)))
  
  (defun buffer-check-marked (buffer)
  
negative.code:      nil)))
  
  (defun buffer-undo (buffer)
    (let ((tlist (buffer-undo-stack buffer)))
      (unless (tlist-empty-p tlist)
        (let ((f (tlist-right tlist))
              (*use-redo-stack* t))
          (setf (car tlist) (butlast (car tlist)))
          (tlist-update tlist)
          (funcall f)))))
  
  (defun buffer-redo (buffer)
  