(define-method-combination chained-or ()
  ((methods *)
   `(or ,@(mapcar #'(lambda (method) `(call-method ,method)) primary))))

(defgeneric read-set (string)
  (:method-combination chained-or))

(defmethod read-set chained-or :antoi ((string string)))
(defmethod read-set chained-or :mintoi ((string string)))

(read-set "123p") ;=> Error: More than one method of type METHODS with the same specializers.