code: def levels(self):
          
          ret = [[] for i in range(self.height)]
          for node in self.subtree:
              ret[node.level - self.level].append(node)
          return ret
docs: Return a list of lists of nodes.
          The outer list is indexed by the level.
          Each inner list contains the nodes at that level,
          in DFS order.
  
          :rtype: list of lists of :class:`~aeneas.tree.Tree`