. Maps each ancestor real path (:class:`str`) to relative
path (:class:`str`).

*on_error* (:class:`~collections.abc.Callable` or :data:`None`)
optionally is the error handler for file-system exceptions.

*follow_links* (:class:`bool`) is whether to walk symbolic links that
resolve to directories.

Yields each entry (:class:`.TreeEntry`).
)