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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def trackers(self):
 """ List of tiers of announce URLs or ``None`` for no trackers A tier is either a single announce URL (:class:`str`) or an :class:`~collections.abc.Iterable` (e.g. :class:`list`) of announce URLs. Setting this property sets or removes ``announce`` and ``announce-list`` in :attr:`metainfo`. ``announce`` is set to the first tracker of the first tier. :raises URLError: if any of the announce URLs is invalid """

        announce_list = self.metainfo.get('announce-list', None)
        if not announce_list:
            announce = self.metainfo.get('announce', None)
            if announce:
                return [[announce]]
        else:
            return announce_list