rot13(a) := simplode(map(ascii, map(lambda([n],
      if (n >= 65 and n <= 77) or (n >= 97 and n <= 109) then n + 13
      elseif (n >= 78 and n <= 90) or (n >= 110 and n <= 122) then n - 13
      else n), map(cint, sexplode(a)))))$

lowercase: "abcdefghijklmnopqrstuvwxyz"$
uppercase: "ABCDEFGHIJKLMNOPQRSTUVWXYZ"$

rot13(lowercase);
"nopqrstuvwxyzabcdefghijklm"

rot13(uppercase);
"NOPQRSTUVWXYZABCDEFGHIJKLM"

rot13("The quick brown fox jumps over the lazy dog");
"Gur dhvpx oebja sbk whzcf bire gur ynml qbt"

rot13(%);
"The quick brown fox jumps over the lazy dog"