public final class Sets {
  private Sets() {}

  /**
   * {@link AbstractSet} substitute without the potentially-quadratic {@code removeAll}
   * implementation.
   */
  abstract static class ImprovedAbstractSet<E extends @Nullable Object> extends AbstractSet<E> {
    @Override
    public boolean removeAll(Collection<?> c) {
      return removeAllImpl(this, c);
    }

    @Override
    public boolean retainAll(Collection<?> c) {

      synchronized (mutex) {
        return delegate().remove(o);
      }
    }

    @Override
    public boolean removeAll(Collection<?> c) {
      synchronized (mutex) {
        return delegate().removeAll(c);
      }
    }

    @Override
    public boolean retainAll(Collection<?> c) {
      synchronized (mutex) {
        return delegate().retainAll(c);
      }

      return get((K) key).remove(value);
    }

    @SuppressWarnings("unchecked")
    @Override
    public Collection<V2> removeAll(@CheckForNull Object key) {
      return transform((K) key, fromMultimap.removeAll(key));
    }

    @Override
    public Collection<V2> replaceValues(@ParametricNullness K key, Iterable<? extends V2> values) {

    }.test();
  }

  public void testRemoveAll() {
    List<String> list = newArrayList("a", "b", "c", "d", "e");
    assertTrue(Iterators.removeAll(list.iterator(), newArrayList("b", "d", "f")));
    assertEquals(newArrayList("a", "c", "e"), list);
    assertFalse(Iterators.removeAll(list.iterator(), newArrayList("x", "y", "z")));
    assertEquals(newArrayList("a", "c", "e"), list);
  }

  public void testRemoveIf() {

    }.test();
  }

  public void testRemoveAll() {
    List<String> list = newArrayList("a", "b", "c", "d", "e");
    assertTrue(Iterators.removeAll(list.iterator(), newArrayList("b", "d", "f")));
    assertEquals(newArrayList("a", "c", "e"), list);
    assertFalse(Iterators.removeAll(list.iterator(), newArrayList("x", "y", "z")));
    assertEquals(newArrayList("a", "c", "e"), list);
  }

  public void testRemoveIf() {

    }
    try {
      values.remove("four");
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      values.removeAll(Collections.singleton("four"));
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      values.retainAll(Collections.singleton("four"));

    }
    try {
      values.remove("four");
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      values.removeAll(Collections.singleton("four"));
      fail("UnsupportedOperationException expected");
    } catch (UnsupportedOperationException expected) {
    }
    try {
      values.retainAll(Collections.singleton("four"));

   * @param elementsToRemove the elements to remove
   * @return {@code true} if any element was removed from {@code iterator}
   */
  @CanIgnoreReturnValue
  public static boolean removeAll(Iterator<?> removeFrom, Collection<?> elementsToRemove) {
    checkNotNull(elementsToRemove);
    boolean result = false;
    while (removeFrom.hasNext()) {
      if (elementsToRemove.contains(removeFrom.next())) {