* {@link TreeSet} or the {@link Map#keySet} of an {@code IdentityHashMap}.
   */
  public static <E extends @Nullable Object> SetView<E> union(
      final Set<? extends E> set1, final Set<? extends E> set2) {
    checkNotNull(set1, "set1");
    checkNotNull(set2, "set2");

    return new SetView<E>() {
      @Override
      public int size() {
        int size = set1.size();

   * 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}.
   *
   * @since 3.0
   */
  public static <E extends @Nullable Object> SetView<E> symmetricDifference(
      final Set<? extends E> set1, final Set<? extends E> set2) {
    checkNotNull(set1, "set1");

	}
	return true
}

// A _Set is a set of elements of some type.
type _Set[Elem comparable] struct {
	m map[Elem]struct{}
}

// _Make makes a new set.
func _Make[Elem comparable]() _Set[Elem] {
	return _Set[Elem]{m: make(map[Elem]struct{})}
}

// Add adds an element to a set.
func (s _Set[Elem]) Add(v Elem) {
	s.m[v] = struct{}{}
}

// Delete removes an element from a set. If the element is not present

	for {
		select {
		case <-ctx.Done():
			return
		case <-timer.C:
			var wg sync.WaitGroup
			for _, set := range s.sets {
				wg.Add(1)
				go func(set *erasureObjects) {
					defer wg.Done()
					if set == nil {
						return
					}
					set.cleanupDeletedObjects(ctx)
				}(set)
			}
			wg.Wait()

			// Reset for the next interval
			timer.Reset(globalAPIConfig.getDeleteCleanupInterval())

                  @Override
                  protected Set<String> create(String[] elements) {
                    Set<String> set1 = Sets.newHashSet(elements);
                    set1.add(samples().e3());
                    Set<String> set2 = Sets.newHashSet(elements);
                    set2.add(samples().e4());
                    return Sets.intersection(set1, set2);
                  }
                })

                  @Override
                  protected Set<String> create(String[] elements) {
                    Set<String> set1 = Sets.newHashSet(elements);
                    set1.add(samples().e3());
                    Set<String> set2 = Sets.newHashSet(elements);
                    set2.add(samples().e4());
                    return Sets.intersection(set1, set2);
                  }
                })

    def "can concatenate attribute to a singleton set"() {
        when:
        def set1 = factory.of(FOO, "foo")
        def set2 = factory.concat(set1, BAR, "bar")

        then:
        set2.keySet() == [FOO, BAR] as Set

        and:
        set2.getAttribute(FOO) == 'foo'
        set2.getAttribute(BAR) == 'bar'
    }

    def "can concatenate attribute to multiple value set"() {
        given:

        CompositeResolvedArtifactSet.of([ResolvedArtifactSet.EMPTY, set, ResolvedArtifactSet.EMPTY]) == set
    }

    def "visits each set in turn"() {
        def set1 = Mock(ResolvedArtifactSet)
        def set2 = Mock(ResolvedArtifactSet)

        def asyncListener = Mock(ResolvedArtifactSet.Visitor)

        when:
        CompositeResolvedArtifactSet.of([set1, set2]).visit(asyncListener)

	expectedSets := sets.NewString("foo", "bar")
	if !expectedSets.Equal(sets.NewString(setList...)) {
		t.Errorf("Unexpected sets mismatch, expected: %v, got: %v", expectedSets, setList)
	}

	// Destroy a given set
	if err := fake.DestroySet(set.Name); err != nil {
		t.Errorf("Unexpected error: %v", err)
	}
	if fake.Sets[set.Name] != nil {
		t.Errorf("Unexpected set: %v", fake.Sets[set.Name])
	}
	if fake.Entries[set.Name] != nil {

    SortedSet<String> set = of("a", "b", "c");
    assertEquals(set, Sets.newTreeSet(asList("a", "b", "c")));
    assertEquals(Sets.newTreeSet(asList("a", "b", "c")), set);
    assertFalse(set.equals(Sets.newTreeSet(asList("a", "b", "d"))));
    assertFalse(Sets.newTreeSet(asList("a", "b", "d")).equals(set));
    assertFalse(set.equals(Sets.newHashSet(4, 5, 6)));
    assertFalse(Sets.newHashSet(4, 5, 6).equals(set));
  }