def bottom_level_series(self) -> Optional[list[Self]]:
        """
        The series containing the bottom-level components of this series in the same
        order as they appear in the series, or None if the series has no hierarchy.

        The returned series is multivariate if there are multiple bottom components.
        """
        return (
            self[[c for c in self.components if c in self.bottom_level_components]]
            if self.has_hierarchy
            else None
        )

        The series containing the bottom-level components of this series in the same
        order as they appear in the series, or None if the series has no hierarchy.

        The returned series is multivariate if there are multiple bottom components.
        
darts/timeseries.py