docs: public static int intersectionSize(DBIDs first, DBIDs second) {
      // If exactly one is a Set, use it as second parameter.
      if(second instanceof SetDBIDs) {
        if(!(first instanceof SetDBIDs)) {
          return internalIntersectionSize(first, second);
        }
      }
      else if(first instanceof SetDBIDs) {
        return internalIntersectionSize(second, first);
      }
      // Both are the same type: both set or both non set.
      // Smaller goes first.
      return first.size() <= second.size() ? internalIntersectionSize(first, second) : internalIntersectionSize(second, first);
    }