docs: private static int internalIntersectionSize(DBIDs first, DBIDs second) {
      second = second.size() > 16 && !(second instanceof SetDBIDs) ? newHashSet(second) : second;
      int c = 0;
      for(DBIDIter it = first.iter(); it.valid(); it.advance()) {
        if(second.contains(it)) {
          c++;
        }
      }
      return c;
    }