anchor.diff:      :initform nil
       :initarg :tail-line
       :accessor buffer-tail-line)
  -   (cache-line
  -    :initform nil
  -    :initarg :cache-line
  -    :accessor buffer-cache-line)
  -   (cache-linum
  -    :initform nil
  -    :initarg :cache-linum
  -    :accessor buffer-cache-linum)
      (mark-p
       :initform nil
       :initarg :mark-p
  
positive.code: 
  (defun buffer-get-line (buffer linum)
    (check-linum buffer linum)
    (let ((line (%buffer-get-line buffer linum)))
      (setf (buffer-cache-linum buffer) linum)
      (setf (buffer-cache-line buffer) line)
      line))
  
  (defun buffer-get-char (buffer linum charpos)
    (let ((line (buffer-get-line buffer linum)))
  
negative.code:         (line (make-line nil nil "")))
      (setf (buffer-head-line buffer) line)
      (setf (buffer-tail-line buffer) line)
  
      (setf (buffer-mark-p buffer) nil)
      (setf (buffer-mark buffer) nil)
      (setf (buffer-keep-binfo buffer) nil)
  