// Don't allow default serialization.
  @GwtIncompatible // java.io.ObjectStreamException
  @J2ktIncompatible
  private void readObjectNoData() throws ObjectStreamException {
    throw new InvalidObjectException("Stream data required");
  }

  @GwtIncompatible // not needed in emulated source.
  @J2ktIncompatible
  private static final long serialVersionUID = -2250766705698539974L;

  @J2ktIncompatible
  private void writeObject(ObjectOutputStream stream) throws IOException {
    stream.defaultWriteObject();
    Serialization.writeMultiset(this, stream);
  }

  @GwtIncompatible // java.io.ObjectInputStream
  @J2ktIncompatible
  private void readObject(ObjectInputStream stream) throws IOException, ClassNotFoundException {
    stream.defaultReadObject();