Object unless := method(if(call evalArgAt(0) not, call evalArgAt(1), call evalArgAt(2)))
That's it. You can now use a new keyword in your scripts:
Io> x := 15; unless(x < 10, "passed", "failed")
Io> x := 5; unless(x < 10, "passed", "failed")
In Common Lisp a similar effect can be achieved by using a macro like this:
(defmacro unless (condition &rest body) `(if (not ,condition) ,@body))
but in contrast to Common Lisp, Io does not use separate constructs to extend the language - what you can do in Lisp using macros, you can do in Io using only its own introspective features.