If name is not in the map, append it with an empty id range set.

def append(self, name):
        """If name is not in the map, append it with an empty id range set."""

        if not isinstance(name, str):
            raise TypeError(
                    "argument 'name' must be a string, not {}".format(
                        name.__class__.__name__
                        )
                    )

        if not name:
            raise ValueError("argument 'name' cannot be empty")

        if not name in self.__map:
            self.__map[name] = IdRangeSet()