def __init__(self, ex_iterables: List[_BaseExamplesIterable]):
        super().__init__()
        self.ex_iterables = ex_iterables
        # TODO(QL): implement iter_arrow

    @property
    def is_typed(self):
        return self.ex_iterables[0].is_typed

    @property
    def features(self):
        return self.ex_iterables[0].features

    def _init_state_dict(self) -> dict:
        self._state_dict = {"ex_iterables": [ex_iterable._init_state_dict() for ex_iterable in self.ex_iterables]}
        return self._state_dict