(define (rot13 s)
(define (char-plus c n)
(integer->char (+ n (char->integer c))))
(define (rot c)
(cond ((char<=? #\a c #\m) (char-plus c 13))
((char<=? #\n c #\z) (char-plus c -13))
((char<=? #\A c #\M) (char-plus c 13))
((char<=? #\N c #\Z) (char-plus c -13))
(else c)))
(list->string (map rot (string->list s))))
(display (rot13 "Cebtenzzvat Cenkvf vf sha!"))