Polymorphism in eieio

Dear list,

I ma trying to use eieio to write code in elisp. But I am stuck with a
problem related to polymorphism. Here is a simple example showing my
problem

(defclass a()
  ((a :initarg :a
      :initform 0))
  "class a")

(defclass b()
  ((b :initarg :b
      :initform 0))
  "class b")

(defmethod nb ((xa a))
  (oref xa :a))

(defmethod nb ((xb b))
  (oref xb :b))

(setq xa (a))

(setq xb (b))

(nb xa) ;;yields 0 as expected

(nb xb) ;;also yields 0

(mapcar 'nb '(xa xb)) ;; yields cl-no-applicable-method (nb xa)

I create two classes a and b and they both have a method nb. When I have
a list made of objects that can be either a's or b's I would expect to
be able to map the method nb on the list, but this fails. Is there a
workaround this problem.

Thanks,

Sylvain.

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