docs: public void read(NutDataInputStream in, long startcode) throws IOException {
      this.startcode = startcode;
      forwardPtr = in.readVarLong();
      if (forwardPtr > 4096) {
        long expected = in.getCRC();
        checksum = in.readInt();
        if (checksum != expected) {
          // TODO This code path has never been tested.
          throw new IOException(
              String.format("invalid header checksum %X want %X", expected, checksum));
        }
      }
  
      in.resetCRC();
      end = in.offset() + forwardPtr - 4; // 4 bytes for footer CRC
    }