Function fac

code/mathematics/intx.test.cpp:18–25

}

intx fac(int n) {
  intx res(1);
  for (int i = 1; i <= n; i++) {
    res = res * i;
  }

  return res;
}

intx gcd(intx a, intx b) {
  return eq(b, intx(0)) ? a : gcd(b, a % b);