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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 start(self):
 """ Given the pipeline topology starts ``Pipers`` in the order input -> output. See ``Piper.start``. ``Pipers`` instances are started in two stages, which allows them to share ``NuMaps``. """

        # top - > bottom of pipeline
        pipers = self.postorder()
        # 
        for piper in pipers:
            piper.start(stages=(0, 1))
        for piper in pipers:
            piper.start(stages=(2,))