Returns a list of all ancestors of all the observed nodes.

        Parameters
        ----------
        obs_nodes_list: string, list-type
            name of all the observed nodes

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)