void main()
{
  int a = 12;

  int fact(int n)
  {
    int fact_tail(int n, int f)
    {
      if (n == 0)
	return f;

      return fact_tail(n - 1, n * f);
    }

    return fact_tail(n, 1);
  }

  println(a, "! = ", fact(a));
}