@Override
    protected Set<V> delegate() {
      return emptySet();
    }
  }

  private static final class AddRejectingList<
          K extends @Nullable Object, V extends @Nullable Object>
      extends ForwardingList<V> {
    @ParametricNullness final K key;

    AddRejectingList(@ParametricNullness K key) {
      this.key = key;
    }

    @Override
    public boolean add(@ParametricNullness V v) {