@GwtCompatible
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

      return map.entrySet().contains(Maps.immutableEntry(key, value));
    }

    @Override
    public Set<V> get(@ParametricNullness K key) {
      return new Sets.ImprovedAbstractSet<V>() {
        @Override
        public Iterator<V> iterator() {
          return new Iterator<V>() {
            int i;

            @Override
            public boolean hasNext() {