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