docs: static protected int countSharedNeighbors(DBIDs neighbors1, DBIDs neighbors2) {
      int intersection = 0;
      DBIDIter iter1 = neighbors1.iter();
      DBIDIter iter2 = neighbors2.iter();
      while(iter1.valid() && iter2.valid()) {
        final int comp = DBIDUtil.compare(iter1, iter2);
        if(comp == 0) {
          intersection++;
          iter1.advance();
          iter2.advance();
        }
        else if(comp < 0) {
          iter1.advance();
        }
        else // iter2 < iter1
        {
          iter2.advance();
        }
      }
      return intersection;
    }