已知整数a,b，假设函数succ（x）=x+1，pred（x）=x-1，不许直接用‘+’、‘-’运算符号，也不许用循环语句，只能利用函数succ（x）和pred（x），试编写计算a+b，a-b的递归函数add（a，b），snb（a,b），并在主程序中验证该函数的正确性。

-Known integers a, b, assuming the function succ (x) = x+1, pred (x) = x-1, allowed the direct use of' +' ,' -'　operational symbol, also not allowed to use the loop, only use the function succ (x) and pred (x), test preparation calculate a+ b, ab recursive function add (a, b), snb (a, b), and in the main program verify the correctness of the function.