docs: private static double erf_of_nonneg(final double x) {
      // The constants that appear below, formatted for easy checking against the book.
      //    a1 = 0.07052 30784
      //    a3 = 0.00927 05272
      //    a5 = 0.00027 65672
      //    a2 = 0.04228 20123
      //    a4 = 0.00015 20143
      //    a6 = 0.00004 30638
      final double a1 = 0.0705230784;
      final double a3 = 0.0092705272;
      final double a5 = 0.0002765672;
      final double a2 = 0.0422820123;
      final double a4 = 0.0001520143;
      final double a6 = 0.0000430638;
      final double x2 = x * x; // x squared, x cubed, etc.
      final double x3 = x2 * x;
      final double x4 = x2 * x2;
      final double x5 = x2 * x3;
      final double x6 = x3 * x3;
      final double sum = ( 1.0
                   + (a1 * x)
                   + (a2 * x2)
                   + (a3 * x3)
                   + (a4 * x4)
                   + (a5 * x5)
                   + (a6 * x6) );
      final double sum2 = sum * sum; // raise the sum to the 16th power
      final double sum4 = sum2 * sum2;
      final double sum8 = sum4 * sum4;
      final double sum16 = sum8 * sum8;
      return (1.0 - (1.0 / sum16));
    }