(define (capitalize-a-string str)
  (if (empty-string? str)
      ""
      (string-append 
        (as-string (capitalize-char (string-ref str 0)))
        (substring str 1 (string-length str)))))

 
(define (upcase-a-string str)
 (list->string (map capitalize-char (string->list str))))

 
(define (downcase-a-string str)
 (list->string (map decapitalize-char (string->list str))))