Iterate items at specified depth.

def items_at(self, depth):
        """
        Iterate items at specified depth.
        """
        if depth < 1:
            yield ROOT, self
        elif depth == 1:
            for key, value in self.items():
                yield key, value
        else:
            for dict_tree in self.values():
                for key, value in dict_tree.items_at(depth - 1):
                    yield key, value