@staticmethod
def _is_cif(string):
    """Test if input string is in CIF format.
        
        :param string: Input string.
        :type string: :py:class:`str` or :py:class:`bytes`
        :return: Input string if in CIF format or False otherwise.
        :rtype: :py:class:`str` or :py:obj:`False` 
        """
    if string[0:5] == u'data_' and u'_entry.id' in string or string[0:5
        ] == b'data_' and b'_entry.id' in string:
        return string
    return False
