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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def levels(self):
 """ 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` """

        ret = [[] for i in range(self.height)]
        for node in self.subtree:
            ret[node.level - self.level].append(node)
        return ret