xfr_writer_add_SOA
xfr_writer_add_SOA(&xw, producer_name, xw.new_serial);
xfr_writer_add_SOA(&xw, producer_name, xw.old_serial);
xfr_writer_add_SOA(&xw, producer_name, xw.new_serial);
xfr_writer_add_SOA(&xw, producer_name, xw.new_serial);