def register(self, event, fn):
        """
        Registers the given function as a handler to be applied
        in response to the the given event.
        """

        # TODO: Can we check the method signature?
        self._handler_dict.setdefault(event, [])
        if fn not in self._handler_dict[event]:
            self._handler_dict[event].append(fn)
Registers the given function as a handler to be applied
        in response to the the given event.