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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 walk_up(self, depth):
 """ Returns a generator yielding all nodes in the tree at a specific depth, or above. Yields nodes starting with leaves and traversing up to the root. :param depth: An integer >= 0 of the depth of nodes to walk up from :return: A generator yielding PolicyTreeNode objects """


        for child in list(self.children):
            if depth != 0:
                for grandchild in child.walk_up(depth - 1):
                    yield grandchild
            yield child