def register(self, observers):
        """
        Concrete method of Subject.register().
        Register observers as an argument to self.observers.
        """

        if isinstance(observers, list) or isinstance(observers, tuple):

            for observer in observers:
                # check whether inhelitance "base.Observer"
                if isinstance(observer, base.Observer):
                    self._observers.append(observer)

                else:
                    raise InhelitanceError(base.Observer.__name__)

        elif isinstance(observers, base.Observer):
            self._observers.append(observers)