* that is not contained in {@code set1}.
   *
   * <p>Results are undefined if {@code set1} and {@code set2} are sets based on different
   * equivalence relations, for example if {@code set1} is a {@link HashSet} and {@code set2} is a
   * {@link TreeSet} or the {@link Map#keySet} of an {@code IdentityHashMap}.
   */
  public static <E extends @Nullable Object> SetView<E> union(

		// request is supported. At the same time, we cannot allow adding
		// two delete markers on top of any object. To avoid this situation,
		// we will sort deletions to execute existing deletion first,
		// then add only one delete marker if requested
		sort.SliceStable(fivs.Versions, func(i, j int) bool {
			return !fivs.Versions[i].Deleted
		})