(defun modify-checker (filename)
  (labels ((mtime () (sb-posix:stat-mtime (sb-posix:stat filename))))
    (let ((last-mtime (mtime)) current-mtime)
      (loop do
           (sleep 3)
           (setf current-mtime (mtime))
           (when (/= current-mtime last-mtime)
             (format t "modified!~%")
             (setf last-mtime current-mtime))))))