code: def store_from(self, last_level_store):
          
          assert isinstance(last_level_store, Cache), \
              "last_level needs to be a Cache object."
          assert last_level_store.store_to is None, \
              "last_level_store must be a last level cache (.store_to is None)."
          self.last_level_store = last_level_store
docs: Set level where to store to.