Es gibt keine größere Illusion als die Meinung,
Sprache sei ein Mittel der Kommunikation zwischen Menschen.
(Elias Canetti)

I wonder wheter any of the "modern" object systems and their reflection APIs can do this:

;; define classes a and b
(defclass a () ())
(defclass b () ())

(defgeneric test (obj))
(defmethod test ((obj a)) 'a-method)
(defmethod test ((obj t)) 't-method)

;; the t-method is called, since b is not subclass of a
(assert (eql 't-method
	     (test (make-instance 'b))))

;; now we add the class a as an additional superclass
 (find-class 'b)
 :direct-superclasses (list (find-class 'a)))

;; and now, the a-method is called, since b is subclass of a
(assert (eql 'a-method
	     (test (make-instance 'b))))