* for the data format.
   */
  static <K extends @Nullable Object, V extends @Nullable Object> void populateMap(
      Map<K, V> map, ObjectInputStream stream, int size)
      throws IOException, ClassNotFoundException {
    for (int i = 0; i < size; i++) {
      @SuppressWarnings("unchecked") // reading data stored by writeMap
      K key = (K) stream.readObject();

   * for the data format.
   */
  static <K extends @Nullable Object, V extends @Nullable Object> void populateMap(
      Map<K, V> map, ObjectInputStream stream, int size)
      throws IOException, ClassNotFoundException {
    for (int i = 0; i < size; i++) {
      @SuppressWarnings("unchecked") // reading data stored by writeMap
      K key = (K) stream.readObject();

  @GwtIncompatible // java.io.ObjectOutputStream
  private void writeObject(ObjectOutputStream stream) throws IOException {
    stream.defaultWriteObject();
    stream.writeObject(keyTypeOrObjectUnderJ2cl);
    Serialization.writeMap(this, stream);
  }

  @SuppressWarnings("unchecked") // reading field populated by writeObject
  @GwtIncompatible // java.io.ObjectInputStream

    stream.defaultWriteObject();
    stream.writeObject(keyTypeOrObjectUnderJ2cl);
    stream.writeObject(valueTypeOrObjectUnderJ2cl);
    Serialization.writeMap(this, stream);
  }

  @SuppressWarnings("unchecked") // reading fields populated by writeObject
  @GwtIncompatible // java.io.ObjectInputStream

    stream.defaultWriteObject();
    stream.writeObject(keyTypeOrObjectUnderJ2cl);
    stream.writeObject(valueTypeOrObjectUnderJ2cl);
    Serialization.writeMap(this, stream);
  }

  @SuppressWarnings("unchecked") // reading fields populated by writeObject
  @GwtIncompatible // java.io.ObjectInputStream

   */
  @GwtIncompatible
  @J2ktIncompatible
    private void writeObject(ObjectOutputStream stream) throws IOException {
    stream.defaultWriteObject();
    Serialization.writeMap(this, stream);
  }

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

   */
  @GwtIncompatible
  @J2ktIncompatible
    private void writeObject(ObjectOutputStream stream) throws IOException {
    stream.defaultWriteObject();
    Serialization.writeMap(this, stream);
  }

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

	for i := range w.buffer {
		delete(w.buffer, i)
	}
	w.wg.Wait()
	return err
}

type writerAt struct {
	w      *io.PipeWriter
	r      *io.PipeReader
	wg     *sync.WaitGroup
	buffer map[int64][]byte
	err    error

	nextOffset int64
	m          sync.Mutex
}

func (w *writerAt) WriteAt(b []byte, offset int64) (n int, err error) {
	w.m.Lock()
	defer w.m.Unlock()