racket

1. let 和 let* 区别
   ``` racket
   // 这是对的
   (let* ([x (random 4)][o (random 4)]
   [diff (number->string (abs (- x o)))])
   (cond [(> x o) (string-append "X wins by " diff)][(> o x) (string-append "O wins by " diff)]
   [else "cat's game"])
   )

// 这是错的
(let ([x (random 4)][o (random 4)]
[diff (number->string (abs (- x o)))])
(cond [(> x o) (string-append "X wins by " diff)][(> o x) (string-append "O wins by " diff)]
[else "cat's game"])
)
```
lets 变量定义后可以直接使用 let 不可以.