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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 drain(self):
 """Walk the BufferTree and reset it when finished. Yields: any: The current node's value. """

        if self.__container_nodes:
            raise ValueError("Attempted to drain without ending all containers.")
        for buf in self.__depth_traverse(self.__root):
            if buf is not None:
                yield buf
        self.__reset()