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 )]
           
           )
          ]
    )
  )