F rot13(string)
   V r = ‘ ’ * string.len
   L(c) string
      r[L.index] = S c
                      ‘a’..‘z’
                         Char(code' (c.code - ‘a’.code + 13) % 26 + ‘a’.code)
                      ‘A’..‘Z’
                         Char(code' (c.code - ‘A’.code + 13) % 26 + ‘A’.code)
                      E
                         c
   R r

print(rot13(‘foo’))
print(rot13(‘sbb’))