go =>
  S = "Big fjords vex quick waltz nymph!",
  println(S),
  println(rot13(S)),
  println(rot13(rot13(S))),
  nl.

% Rot 13 using a map
rot13(S) = S2 =>
  lower(Lower),
  upper(Upper),
  M = create_map(Lower, Upper),
  % If a char is not in a..zA..z then show it as it is.
  S2 := [M.get(C,C) : C in S].

create_map(Lower, Upper) = M =>
  M = new_map(),
  Len = Lower.length,
  LDiv := Lower.length div 2,
  foreach(I in 1..Len)
    II = (LDiv+I) mod Len,
    if II == 0 then II := Len end,
    M.put(Upper[I],Upper[II]),
    M.put(Lower[I],Lower[II])
  end.

lower("abcdefghijklmnopqrstuvwxyz").
upper("ABCDEFGHIJKLMNOPQRSTUVWXYZ").