docs: public static double normF(double[][] elements) {
      double f = 0;
      for(int i = 0; i < elements.length; i++) {
        final double[] row = elements[i];
        for(int j = 0; j < row.length; j++) {
          final double v = row[j];
          f += v * v;
        }
      }
      // TODO: detect overflow, fall back to slower hypot()
      return FastMath.sqrt(f);
    }