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