docs: 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)