Add a sequence of items to the end of the list

        :param iter: The iterable of items to add.
        :param parent: The node to add the items as a child of, or None for
                       top-level nodes.

def extend(self, iter, parent=None):
        """Add a sequence of items to the end of the list

        :param iter: The iterable of items to add.
        :param parent: The node to add the items as a child of, or None for
                       top-level nodes.
        """
        for item in iter:
            self.append(item, parent)