@abc.abstractmethod<EOL><INDENT>def get_zeptoseconds(self):<DEDENT>
return<EOL>
Gets the number of zeptoseconds of the attosecond 0-999.

        A zeptosecond is one sextillionth of a second.

        :return: the zeptoseconds of the attosecond
        :rtype: ``cardinal``


        *compliance: mandatory -- This method must be implemented.*