@staticmethod
def is_sdfile(string):
    """Test if input string is in ``SDfile`` format.

        :param string: Input string.
        :type string: :py:class:`str` or :py:class:`bytes`.
        :return: Input string if in ``SDfile`` format or False otherwise.
        :rtype: :py:class:`str` or :py:obj:`False`.
        """
    try:
        string = string.decode('utf-8')
    except AttributeError:
        pass
    if '$$$$\n' in string:
        return True
    return False
