docs: private static ModifiableDBIDs internalIntersection(DBIDs first, DBIDs second) {
      second = second.size() > 16 && !(second instanceof SetDBIDs) ? newHashSet(second) : second;
      ModifiableDBIDs inter = newHashSet(first.size());
      for(DBIDIter it = first.iter(); it.valid(); it.advance()) {
        if(second.contains(it)) {
          inter.add(it);
        }
      }
      return inter;
    }