def _latest_common_snapshot(some, others):
    """
    Pick the most recent snapshot that is common to two snapshot lists.

    :param list some: One ``list`` of ``Snapshot`` instances to consider,
        ordered from oldest to newest.

    :param list others: Another ``list`` of ``Snapshot`` instances to consider,
        ordered from oldest to newest.

    :return: The ``Snapshot`` instance which occurs closest to the end of both
        ``some`` and ``others`` If no ``Snapshot`` appears in both, ``None`` is
        returned.
    """
    others_set = set(others)
    for snapshot in reversed(some):
        if snapshot in others_set:
            return snapshot
    return None