} else if (unfiltered instanceof SetMultimap) {
      return new AddRejectingSet<>(key);
    } else {
      return new AddRejectingList<>(key);
    }
  }

  static class AddRejectingSet<K extends @Nullable Object, V extends @Nullable Object>
      extends ForwardingSet<V> {
    @ParametricNullness final K key;

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