:unset +s +t
-- tests for :def, :undef
let void m = m >> return ""
let echo s = void (putStrLn s)
:def echo echo
:echo hello, world!

let echo s = return (":! echo " ++ map Char.toUpper s)
:echo hello, world!
:def echo echo
:undef echo
:def echo echo
:echo hello, world!