code: def load_to(self, last_level_load):
          
          assert isinstance(last_level_load, Cache), \
              "last_level needs to be a Cache object."
          assert last_level_load.load_from is None, \
              "last_level_load must be a last level cache (.load_from is None)."
          self.last_level_load = last_level_load
docs: Set level where to load from.