code: func (a *AddBackDep) RollForward(c context.Context) (muts []tumble.Mutation, err error) {
  	bdg, bd := a.Dep.Back(c)
  	err = ds.Get(c, bdg)
  	if err == ds.ErrNoSuchEntity {
  		err = ds.Put(c, bdg)
  	}
  	if err != nil {
  		return
  	}
  
  	bd.Propagated = bdg.AttemptFinished
  
  	err = ds.Put(c, bd)
  
  	if a.NeedsAck && bdg.AttemptFinished {
  		muts = append(muts, &AckFwdDep{a.Dep})
  	}
  	return
  }
docs: // RollForward implements tumble.Mutation.