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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 finalize(self):
 """ Look at our tiers and setup the final data flow. Once this is run a cell can not be modified again. """

        self.assertNotFinalized()
        starters = []
        finishers = []
        for x in self.tiers:
            if not x.sources:
                starters.append(x)
            if not x.dests:
                finishers.append(x)
        self.add_tier(self.output_feed, source=finishers)
        self.coord.setup_wrap(self)
        self.finalized = True
        return starters