Setter for **self.__default_node** attribute.

        :param value: Attribute value.
        :type value: AbstractCompositeNode or GraphModelNode

def default_node(self, value):
        """
        Setter for **self.__default_node** attribute.

        :param value: Attribute value.
        :type value: AbstractCompositeNode or GraphModelNode
        """

        if value is not None:
            assert issubclass(value, AbstractCompositeNode), \
                "'{0}' attribute: '{1}' is not a '{2}' subclass!".format(
                    "default_node", value, AbstractCompositeNode.__name__)
        self.__default_node = value