(defclass b () ())
(defmethod m1 ((_ b)) "m1")
(defclass c (b) ())
(setf obj (make-instance 'c))
(format t "~A~%" (m1 obj)) ;;=> m1

(defclass a () ())
(defclass b (a) ())
(defmethod m2 ((_ a)) "m2")
(format t "~A~%" (m1 obj)) ;;=> m1
(format t "~A~%" (m2 obj)) ;;=> m2