def ancestry(mention):
    """ Compute the ancestry of a mention.

    We follow the definition of the ancestry by Durrett and Klein (2013). For
    more information, have a look at their paper:

    Greg Durrett and Dan Klein. Easy Victories and Uphill Battles in
    Coreference Resolution. In Proceedings of EMNLP 2013.
    http://anthology.aclweb.org/D/D13/D13-1203.pdf

    Args:
        mention (Mention): A mention.

    Returns:
        The tuple ('ancestry', ANCESTRY), where ANCESTRY is the ancestry of
        the mention.
    """
    return "ancestry", mention.attributes["ancestry"]