def (rot13 s)
  (as string
      (map rot13
           (as list s)))

Alphabet <- "abcdefghijklmnopqrstuvwxyz"
def (rot13 c) :case (and string?.c len.c=1)
  if ("a" <= c <= "z")
       let idx (pos c Alphabet)
         Alphabet (idx+13 % 26)
     ("A" <= c <= "Z")
       (downcase.c -> rot13 -> upcase)
     :else
       c