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