code: def list_things_at(self, location, tclass=Thing):
          "Return all things exactly at a given location."
          return [thing for thing in self.things
                  if thing.location == location and isinstance(thing, tclass)f list_things_at(self, location, tclass=Thing):
          "Return all things exactly at a given location."
          return [thing for thing in self.things
                  if thing.location == location and isinstance(thing, tclass)]
docs: Return all things exactly at a given location.