Do nothing if given name refers to a powernode in given graph.
        Raise a ValueError in any other case.

def assert_powernode(self, name:str) -> None or ValueError:
        """Do nothing if given name refers to a powernode in given graph.
        Raise a ValueError in any other case.

        """
        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))