return (Collector) ONLY_ELEMENT;
  }

  /**
   * This atrocity is here to let us report several of the elements in the stream if there were more
   * than one, not just two.
   */
  private static final class ToOptionalState {
    static final int MAX_EXTRAS = 4;

    @CheckForNull Object element;
    List<Object> extras;

    ToOptionalState() {