@ParametricNullness
      E transform(Entry<E> entry) {
        return entry.getElement();
      }
    };
  }

  abstract static class ElementSet<E extends @Nullable Object> extends Sets.ImprovedAbstractSet<E> {
    abstract Multiset<E> multiset();

    @Override
    public void clear() {
      multiset().clear();
    }

    @Override
    public boolean contains(@CheckForNull Object o) {

      @ParametricNullness
      E transform(Entry<E> entry) {
        return entry.getElement();
      }
    };
  }

  abstract static class ElementSet<E extends @Nullable Object> extends Sets.ImprovedAbstractSet<E> {
    abstract Multiset<E> multiset();

    @Override
    public void clear() {
      multiset().clear();
    }

    @Override
    public boolean contains(@CheckForNull Object o) {

      self.put(entry.getKey(), entry.getValue());
    }
  }

  static class KeySet<K extends @Nullable Object, V extends @Nullable Object>
      extends Sets.ImprovedAbstractSet<K> {
    @Weak final Map<K, V> map;

    KeySet(Map<K, V> map) {
      this.map = checkNotNull(map);
    }

    Map<K, V> map() {
      return map;
    }

    @Override

      self.put(entry.getKey(), entry.getValue());
    }
  }

  static class KeySet<K extends @Nullable Object, V extends @Nullable Object>
      extends Sets.ImprovedAbstractSet<K> {
    @Weak final Map<K, V> map;

    KeySet(Map<K, V> map) {
      this.map = checkNotNull(map);
    }

    Map<K, V> map() {
      return map;
    }

    @Override

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

            @Override
            public boolean hasNext() {