docs: def _ensure_node(node: Union[str, AbstractNode]) -> AbstractNode:
      """Ensure to be node.
  
      If ``node`` is string, convert it to ``Text`` node.
      """
      if isinstance(node, str):
          return Text(node)
      elif isinstance(node, Node):
          return node
      else:
          raise TypeError('Invalid type to append: {}'.format(node))