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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def write_additional(self, productversion, channel):
 """Write the additional information to the MAR header. Args: productversion (str):
 product and version string channel (str):
 channel string """

        self.fileobj.seek(self.additional_offset)
        extras = extras_header.build(dict(
            count=1,
            sections=[dict(
                channel=six.u(channel),
                productversion=six.u(productversion),
                size=len(channel) + len(productversion) + 2 + 8,
                padding=b'',
            )],
        ))

        self.fileobj.write(extras)
        self.last_offset = self.fileobj.tell()