function rot13 S
   repeat with i = 1 to length(S)
      get chartonum(char i of S)
      if it < 65 or it > 122 or (it > 90 and it < 97) then next repeat
      put char it - 64 of "NOPQRSTUVWXYZABCDEFGHIJKLM      nopqrstuvwxyzabcdefghijklm" into char i of S
   end repeat
   return S
end rot13