public static BigInteger Factorial(int n){
    require n >= 0;                                 
    BigInteger res = 1;                             
    for(int i = 1; i <= n; i++)                     
      res = res * i;
    return res;
    ensure res = n * n-1 * ... * 1;  
  }  

 
A precondition: It is required
that n is non-negative. Must be
checked by the caller.