🚀 go-pugleaf

I have a question concerning creating a method applicable to one of
two classes.

Suppose I have two classes A and B. These two classes are distinct.
They happen to share similar instance variables, but it is not
possible to factor this out to a mixin, since I can't redefine A.

I want to write a method which is applicable to either. The cleanest way to
express it is:

(deftype C () '(or A B))

(defmethod common ((instance C))
 ...)

This doesn't work. (can't find class C)

My only two choices seem to be to either replicate the code, or to factor the
body of the definition of common into a defun which is called by both.

1-

(defmethod common ((instance A))
  the code)
(defmethod common ((instance B))
  the same code)

2-

(defmethod common ((instance A))
  (the-code instance))
(defmethod common ((instance B))
  (the-code instance))
(defun the-code (instance)
  the code)

So the question is... Is there a more elegant way to do this?

-alan