docs: public static double quantile(double[] data, int begin, int end, double quant) {
      final int length = end - begin;
      assert (length > 0) : "Quantile on empty set?";
      // Integer division is "floor" since we are non-negative.
      final double dleft = begin + (length - 1) * quant;
      final int ileft = (int) Math.floor(dleft);
      final double err = dleft - ileft;
  
      quickSelect(data, begin, end, ileft);
      if(err <= Double.MIN_NORMAL) {
        return data[ileft];
      }
      else {
        quickSelect(data, ileft + 1, end, ileft + 1);
        // Mix:
        double mix = data[ileft] + (data[ileft + 1] - data[ileft]) * err;
        return mix;
      }
    }