code: def at(self, step):
          
          return TimeMachine(
              self.uid,
              step = step,
              info = copy.deepcopy(self.info)
          )
docs: Return a TimeMachine for the same object at a different time.
  
          Takes an integer argument representing the id field of an Action.
          Returns the TimeMachine at the time of that Action. (Less ambiguously:
          at the time right after the Action.