(fn rot13 [s]
  (let [a :ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
        b :NOPQRSTUVWXYZABCDEFGHIJKLMnopqrstuvwxyzabcdefghijklm]
    (s:gsub :%a #(b:sub (a:find $)))))