(*
        rot13: string -> string
        rot13(s) => rot13 encoded version of s
*)
fun rot13 (s:string) =
    let
        fun rot13_char (c:char) =       (* rot13 a single char *)
            if (c >= #"A" andalso c <= #"N") orelse
                    (c >= #"a" andalso c <= #"n") then chr(ord(c) + 13)
            else if (c >= #"O" andalso c <= #"Z") orelse
                    (c >= #"o" andalso c <= #"z") then chr(ord(c) - 13)
            else c

        fun rot13_clist [] : char list = []     (* empty set *)
            | rot13_clist (c::cs) = rot13_char(c) :: rot13_clist(cs)
    in
        implode(rot13_clist(explode(s)))
    end