V output(@ParametricNullness K key, @ParametricNullness V value) {
        return value;
      }
    };
  }

  @Override
  Spliterator<V> valueSpliterator() {
    return CollectSpliterators.flatMap(
        map.values().spliterator(), Collection::spliterator, Spliterator.SIZED, size());
  }

  /*
   * TODO(kevinb): should we copy this javadoc to each concrete class, so that