A shortcut decorator that invokes :func:`.command` and adds it to
        the internal command list via :meth:`~.GroupMixin.add_command`.

def command(self, *args, **kwargs):
        """A shortcut decorator that invokes :func:`.command` and adds it to
        the internal command list via :meth:`~.GroupMixin.add_command`.
        """
        def decorator(func):
            kwargs.setdefault('parent', self)
            result = command(*args, **kwargs)(func)
            self.add_command(result)
            return result

        return decorator