code: def assert_powernode(self, name:str) -> None or ValueError:
          
          if name not in self.inclusions:
              raise ValueError("Powernode '{}' does not exists.".format(name))
          if self.is_node(name):
              raise ValueError("Given name '{}' is a node.".format(name))
docs: Do nothing if given name refers to a powernode in given graph.
          Raise a ValueError in any other case.