anchor.diff:  (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-undo-size buffer) 0)
  -  (setf (buffer-undo-stack buffer) nil)
  +  (setf (buffer-edit-history buffer) (make-array 0 :adjustable t :fill-pointer 0))
     (setf (buffer-redo-stack buffer) nil)
     nil)
   
  
positive.code: 
  (defvar *undo-modes* '(:edit :undo :redo))
  (defvar *undo-mode* :edit)
  
  (defun make-buffer (name &key temporary read-only-p (enable-undo-p t)
                                (syntax-table (fundamental-syntax-table)))
  
negative.code:   (buffer-mark-cancel buffer))
  
  (defun push-undo-stack (buffer elt)
    (vector-push-extend elt (buffer-edit-history buffer)))
  
  (defun push-redo-stack (buffer elt)
    (push elt (buffer-redo-stack buffer)))
  