* {@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();

        .testForwarding(
            Set.class,
            new Function<Set, Set>() {
              @Override
              public Set apply(Set delegate) {
                return wrap(delegate);
              }
            });
  }

  public void testEquals() {
    Set<String> set1 = ImmutableSet.of("one");
    Set<String> set2 = ImmutableSet.of("two");
    new EqualsTester()

              }
            });
  }

  public void testEquals() {
    SortedMultiset<String> set1 = ImmutableSortedMultiset.of("one");
    SortedMultiset<String> set2 = ImmutableSortedMultiset.of("two");
    new EqualsTester()
        .addEqualityGroup(set1, wrap(set1), wrap(set1))
        .addEqualityGroup(set2, wrap(set2))
        .testEquals();
  }

    assertTrue(set.containsAll(Sets.newTreeSet(asList("a", "b", "f"))));
    assertFalse(set.containsAll(Sets.newTreeSet(asList("d"))));
    assertFalse(set.containsAll(Sets.newTreeSet(asList("z"))));
    assertFalse(set.containsAll(Sets.newTreeSet(asList("b", "d"))));
    assertFalse(set.containsAll(Sets.newTreeSet(asList("f", "d", "a"))));
  }

  }

  /**
   * Construct a new {@link java.util.Set} that is the intersection of the given sets.
   *
   * @deprecated Use {@link com.google.common.collect.Sets#intersection(Set, Set)} instead.
   */
  @Deprecated
  public static <T> Set<T> intersection(Set<? extends T> set1, Set<? extends T> set2) {
    Set<T> result = copyToSet(set1);
    result.retainAll(set2);
    return result;
  }

  private FeatureUtil() {}

        .testForwarding(
            Set.class,
            new Function<Set, Set>() {
              @Override
              public Set apply(Set delegate) {
                return wrap(delegate);
              }
            });
  }

  public void testEquals() {
    Set<String> set1 = ImmutableSet.of("one");
    Set<String> set2 = ImmutableSet.of("two");
    new EqualsTester()

              }
            });
  }

  public void testEquals() {
    SortedSet<String> set1 = ImmutableSortedSet.of("one");
    SortedSet<String> set2 = ImmutableSortedSet.of("two");
    new EqualsTester()
        .addEqualityGroup(set1, wrap(set1), wrap(set1))
        .addEqualityGroup(set2, wrap(set2))
        .testEquals();
  }

  private static <T> SortedSet<T> wrap(final SortedSet<T> delegate) {

              }
            });
  }

  public void testEquals() {
    SortedSet<String> set1 = ImmutableSortedSet.of("one");
    SortedSet<String> set2 = ImmutableSortedSet.of("two");
    new EqualsTester()
        .addEqualityGroup(set1, wrap(set1), wrap(set1))
        .addEqualityGroup(set2, wrap(set2))
        .testEquals();
  }

  private static <T> SortedSet<T> wrap(final SortedSet<T> delegate) {

              }
            });
  }

  public void testEquals() {
    NavigableSet<String> set1 = ImmutableSortedSet.of("one");
    NavigableSet<String> set2 = ImmutableSortedSet.of("two");
    new EqualsTester()
        .addEqualityGroup(set1, wrap(set1), wrap(set1))
        .addEqualityGroup(set2, wrap(set2))
        .testEquals();
  }

  private static <T> NavigableSet<T> wrap(final NavigableSet<T> delegate) {

              }
            });
  }

  public void testEquals() {
    SortedMultiset<String> set1 = ImmutableSortedMultiset.of("one");
    SortedMultiset<String> set2 = ImmutableSortedMultiset.of("two");
    new EqualsTester()
        .addEqualityGroup(set1, wrap(set1), wrap(set1))
        .addEqualityGroup(set2, wrap(set2))
        .testEquals();
  }