code: func (e *EnsureAttempt) RollForward(c context.Context) (muts []tumble.Mutation, err error) {
  	a := model.MakeAttempt(c, e.ID)
  	if err = ds.Get(c, a); err != ds.ErrNoSuchEntity {
  		return
  	}
  
  	if err = ds.Put(c, a); err != nil {
  		logging.WithError(err).Errorf(logging.SetField(c, "id", e.ID), "in put")
  	}
  	muts = append(muts, &ScheduleExecution{e.ID})
  	return
  }
docs: // RollForward implements tumble.Mutation.