By the time we're just folding in clusters, there's no need to maintain
        self.INSTANCES and self.clusters, so we just call this method

def do_static_merge(cls, c_source, c_target):
        """By the time we're just folding in clusters, there's no need to maintain
        self.INSTANCES and self.clusters, so we just call this method
        """
        c_target.extend(c_source)
        c_source.parent = c_target.parent
        cls.CLUSTERS.remove(c_source)
        for m in c_source.mentions:
            cls.MENTION_TO_CLUSTER[m] = c_target