code: def add_tier(self, coro, **kwargs):
          
          self.assertNotFinalized()
          assert asyncio.iscoroutinefunction(coro)
          tier = self.Tier(self, coro, **kwargs)
          self.tiers.append(tier)
          self.tiers_coro_map[coro] = tier
          return tier
docs: Add a coroutine to the cell as a task tier.  The source can be a
          single value or a list of either `Tier` types or coroutine functions
          already added to a `Tier` via `add_tier`.