import Char

rot13 :: String -> String
rot13 = map (chr . rot . ord)
    where
      rot o | o `inn` (65, 90) = shift 65 o
      rot o | o `inn` (97, 122) = shift 97 o
      rot o = o
      shift n o = ((o-n+13)`mod`26)+n
      i `inn` (x, y) = i >= x && i <= y