docs: 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