define-interface :definition example

Is there an example of using :definition in an interface?

My (apparently wrong) understanding of this (with :interface-variable) 
is that any callbacks with the :callback-type :interface will instead 
use the variable value instead. So, for example:

(define-interface foo ()
   ((bar :initarg :bar))
   (:menus
    (test "Test" ("Run" :callback 'test :callback-type :interface)))
   (:menu-bar test)
   (:definition :interface-variable bar))

(defclass bar () ())

(defmethod test ((foo foo)) (display-message "foo"))
(defmethod test ((bar bar)) (display-message "bar"))

If I display an instance of 'foo with :bar set to an instance of 'bar 
and click on the menu item, the message displayed is "foo" and not "bar".

So, I'm curious, what is :interface-variable supposed to do? :-)

Jeff M.

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