def write_additional(self, productversion, channel):
self.fileobj.seek(self.additional_offset)<EOL>extras = extras_header.build(dict(<EOL>count=<NUM_LIT:1>,<EOL>sections=[dict(<EOL>channel=six.u(channel),<EOL>productversion=six.u(productversion),<EOL>size=len(channel) + len(productversion) + <NUM_LIT:2> + <NUM_LIT:8>,<EOL>padding=b'<STR_LIT>',<EOL>)],<EOL>))<EOL>self.fileobj.write(extras)<EOL>self.last_offset = self.fileobj.tell()<EOL>
Write the additional information to the MAR header.

        Args:
            productversion (str): product and version string
            channel (str): channel string