return new HashSet<>();
  }

  /**
   * Creates a <i>mutable</i> {@code HashSet} instance initially containing the given elements.
   *
   * <p><b>Note:</b> if elements are non-null and won't be added or removed after this point, use
   * {@link ImmutableSet#of()} or {@link ImmutableSet#copyOf(Object[])} instead. If {@code E} is an
   * {@link Enum} type, use {@link EnumSet#of(Enum, Enum[])} instead. Otherwise, strongly consider

    List<String> alreadyThere = Lists.newArrayList("already", "there");
    List<String> freshlyAdded = Lists.newArrayList("freshly", "added");

    boolean changed = Iterators.addAll(alreadyThere, freshlyAdded.iterator());

    assertThat(alreadyThere).containsExactly("already", "there", "freshly", "added");
    assertTrue(changed);
  }

  public void testAddAllToSet() {