code: public OggPacketWriter getPacketWriter(int sid) {
          if(!writing) {
              throw new IllegalStateException("Can only write to a file opened with an OutputStream");
          }
          seenSIDs.add(sid);
          return new OggPacketWriter(this, sid);
      }
docs: Creates a new Logical Bit Stream in the file,
  and returns a Writer for putting data
  into it.