Bulk ``register_item``.

        Args:
            items (iterable[Tree]):
                Sequence of nodes to be registered as children.

def register_items(self, items):
        """Bulk ``register_item``.

        Args:
            items (iterable[Tree]):
                Sequence of nodes to be registered as children.
        """
        for item in items:
            item.set_parent(self)
        self.items.extend(items)