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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 failover_limitation(self):
 """Returns reason why this node can't promote or None if everything is ok."""

        if not self.reachable:
            return 'not reachable'
        if self.tags.get('nofailover', False):
            return 'not allowed to promote'
        if self.watchdog_failed:
            return 'not watchdog capable'
        return None