Returns True if this node is on *some* list.

        A node is not on any list if it is linked to itself, or if it
        does not have the next and/prev attributes at all.

def node_is_on_list(self, node):
        """Returns True if this node is on *some* list.

        A node is not on any list if it is linked to itself, or if it
        does not have the next and/prev attributes at all.
        """
        next = self.node_next(node)
        if next == node or next is None:
            assert(self.node_prev(node) is next)
            return False

        return True