anchor.diff:    point)
   
   (defun blank-line-p (point)
  +  "`point`のある行が空白だけなら、その空白の個数、それ以外ならnilを返します。"
     (let ((string (line-string point))
           (eof-p (last-line-p point))
           (count 0))
  
positive.code:         (delete-point mark)))))
  
  (defun insert-buffer (point buffer)
    (loop :for line := (point-line (buffer-start-point buffer)) :then (line-next line)
          :while line
          :do (insert-string point (line-str line))
  
negative.code:                (line-offset (buffer-end point) (- line-number nlines)))))))
  
  (defun check-marked ()
    "`current-buffer`内でマークが置かれていなければ`editor-error`を起こします。"
    (unless (buffer-mark (current-buffer))
      (editor-error "Not mark in this buffer")))
  
  