anchor.diff:      (setf (%buffer-keep-binfo buffer) nil)
       (setf (buffer-nlines buffer) 1)
       (setf (buffer-%modified-p buffer) 0)
  -    (setf (buffer-undo-size buffer) 0)
  -    (setf (buffer-undo-stack buffer) nil)
       (setf (buffer-redo-stack buffer) nil)
       (setf (buffer-variables buffer) (make-hash-table :test 'equal))
       (let ((line (make-line nil nil "")))
  
positive.code:   (buffer-mark-cancel buffer))
  
  (defun push-undo-stack (buffer elt)
    (incf (buffer-undo-size buffer))
    (push elt (buffer-undo-stack buffer)))
  
  (defun push-redo-stack (buffer elt)
    (push elt (buffer-redo-stack buffer)))
  
negative.code: (defun buffer-disable-undo (buffer)
    @lang(:jp "`buffer`のアンドゥを無効にしてアンドゥ用の情報を空にします。")
    (setf (buffer-%enable-undo-p buffer) nil)
    (setf (buffer-edit-history buffer) (make-array 0 :adjustable t :fill-pointer 0))
    (setf (buffer-redo-stack buffer) nil)
    nil)
  
  