code: def as_tree(self):
          "Return the nodes as a list of lists."
          if self._nodes is None:
              return [self]
          ret = [self]
          for node in self._nodes:
              ret.append(node.as_tree())
          return ref as_tree(self):
          "Return the nodes as a list of lists."
          if self._nodes is None:
              return [self]
          ret = [self]
          for node in self._nodes:
              ret.append(node.as_tree())
          return ret
docs: Return the nodes as a list of lists.