Racket- list

上段代码， 计算列表是否包含某个值

;; contains? : list-of-symbol  symbol -> true;; 计算表中是否包含给定的symbol;; (contains? (cons 'a (cons 'b empty)) 'a) =true(define (contains? a-list-of-symbol a-symbol )  (cond         [(empty? a-symbol) false]        [else (cond                         [(symbol=? a-symbol (first a-list-of-symbol )) true]                       [else (contains? (rest a-list-of-symbol) a-symbol )]                      )          ]    )  )