Subclasses

Thanx for your work.
I'm using clg for a project running with SBCL 0.9. (You've ever heard of 
it, it's Open Music)
My first question would be:
Is it possible in my lisp code to define subclasses for "gtk classes", 
for example a subclass for window, and to make instances of this class.  
For the moment, I make it work using the "change-class" function on my 
"gtk instance". (I know, it's quite not "beautiful") :

(defclass my-window (gtk::window) ())
(make-instance 'my-window)

 >Type not registered: my-window
 > [Condition of type simple-error]

I hope this question is not too "easy".
Thanx anyway.

Ephrem


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