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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 _get_ancestors_of(self, obs_nodes_list):
 """ Returns a list of all ancestors of all the observed nodes. Parameters obs_nodes_list: string, list-type name of all the observed nodes """

        if not obs_nodes_list:
            return set()
        return set(obs_nodes_list) | set(self.parent_node)