void begin_large_parameter () {
      if (request_number > 1) {
        // Update the length fields before sending the previous request.
        int index = out.index;
        int ni = out.index - 16;
        int pi = RequestOutputStream.pad (ni);
        out.index = 2;
        out.write_int16 (4 + (ni + pi) / 4);
        out.index = 12;
        out.write_int32 (ni);
        out.index = index;
      }

      request_number++;

      out.begin_request(glx.major_opcode, 2, 0); // Length written later.
      out.write_int32 (tag);
      out.write_int16 (request_number);
      out.write_int16 (request_total);
      out.write_int32 (0); // ni, written later.
    }