code: def write_additional(self, productversion, channel):
          
          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()
docs: Write the additional information to the MAR header.
  
          Args:
              productversion (str): product and version string
              channel (str): channel string