code: def add_time_step(self, **create_time_step_kwargs):
      
      ts = time_step.TimeStep.create_time_step(**create_time_step_kwargs)
      assert isinstance(ts, time_step.TimeStep)
      self._time_steps.append(ts)
docs: Creates a time-step and appends it to the list.
  
      Args:
        **create_time_step_kwargs: Forwarded to
          time_step.TimeStep.create_time_step.