* remain as implementation details.
   */
  @J2ktIncompatible // serialization
  private static class SerializedForm<K, V> extends ImmutableMap.SerializedForm<K, V> {
    private final Comparator<? super K> comparator;

    SerializedForm(ImmutableSortedMap<K, V> sortedMap) {
      super(sortedMap);
      comparator = sortedMap.comparator();
    }

    @Override

  }

  @Override
  @J2ktIncompatible // serialization
  @GwtIncompatible // serialization
  Object writeReplace() {
    return SerializedForm.create(this, cellRowIndices, cellColumnIndices);
  }

    ImmutableSet<SomeEnum> original = Sets.immutableEnumSet(SomeEnum.A, SomeEnum.B);
    int handleOffset = 6;
    byte[] serializedForm = serializeWithBackReference(original, handleOffset);
    ObjectInputStream in = new ObjectInputStream(new ByteArrayInputStream(serializedForm));

    ImmutableSet<?> deserialized = (ImmutableSet<?>) in.readObject();
    EnumSet<?> delegate = (EnumSet<?>) in.readObject();

  }

  private Object writeReplace() {
    return new SerialForm<T>(this);
  }

  private void readObject(ObjectInputStream stream) throws InvalidObjectException {
    throw new InvalidObjectException("Use SerializedForm");
  }

  private static class SerialForm<T extends @Nullable Object> implements Serializable {
    final long[] data;
    final int numHashFunctions;
    final Funnel<? super T> funnel;