def write_header(self):
self.fileobj.seek(<NUM_LIT:0>)<EOL>header = mar_header.build(dict(index_offset=self.last_offset))<EOL>self.fileobj.write(header)<EOL>
Write the MAR header to the file.

        The MAR header includes the MAR magic bytes as well as the offset to
        where the index data can be found.