docs: public static double Phi(double z) {
          if (z < -8.0) return 0.0;
          if (z >  8.0) return 1.0;
          double sum = 0.0, term = z;
          for (int i = 3; sum + term != sum; i += 2) {
              sum  = sum + term;
              term = term * z * z / i;
          }
          return 0.5 + sum * phi(z);
      }