* know that calling {@code clone()} during deserialization will return an
   * object that no one else has a reference to, allowing us to guarantee
   * immutability. Hence, we support only {@link EnumSet}.
   */
  private final transient EnumSet<E> delegate;

  private ImmutableEnumSet(EnumSet<E> delegate) {
    this.delegate = delegate;
  }

  @Override

   * know that calling {@code clone()} during deserialization will return an
   * object that no one else has a reference to, allowing us to guarantee
   * immutability. Hence, we support only {@link EnumSet}.
   */
  private final transient EnumSet<E> delegate;

  private ImmutableEnumSet(EnumSet<E> delegate) {
    this.delegate = delegate;
  }

  @Override