class SplitMap {

  public long getCardinality() {
    return index.reduceLong(0L, Container::getCardinality, (x, y) -> x + y);
  }

  public  SplitMap(PrefixIndex<Container> index, KeyInvolution involution);
  public  SplitMap(PrefixIndex<Container> index);
  public  SplitMap(KeyInvolution involution);

}