code: def start_container(self):
          
          self.__container_lengths.append(self.current_container_length)
          self.current_container_length = 0
          new_container_node = _Node()
          self.__container_node.add_child(new_container_node)
          self.__container_nodes.append(self.__container_node)
          self.__container_node = new_container_node
docs: Add a node to the tree that represents the start of a container.
  
          Until end_container is called, any nodes added through add_scalar_value
          or start_container will be children of this new node.