docs: protected static int partition(Instances insts, int[] index, int attidx, int l, int r) {
      
      double pivot = insts.instance(index[(l + r) / 2]).value(attidx);
      int help;
  
      while (l < r) {
        while ((insts.instance(index[l]).value(attidx) < pivot) && (l < r)) {
          l++;
        }
        while ((insts.instance(index[r]).value(attidx) > pivot) && (l < r)) {
          r--;
        }
        if (l < r) {
          help = index[l];
          index[l] = index[r];
          index[r] = help;
          l++;
          r--;
        }
      }
      if ((l == r) && (insts.instance(index[r]).value(attidx) > pivot)) {
        r--;
      } 
  
      return r;
    }