S-99-80

 (define (inverte x) 
  
   (define (f n) 
     (string->number 
      (list->string 
       (reverse 
        (string->list 
         (number->string n)))))) 
  
   (if (>= x 0) 
       (f x) 
       (- (f (abs x))))) 
  
 (inverte 31415) ; 51413