code: func recentlyFinishedSet(c context.Context, jobID string) *invocationIDSet {
  	return &invocationIDSet{
  		Set: dsset.Set{
  			ID:              "finished:" + jobID,
  			ShardCount:      8,
  			TombstonesRoot:  datastore.KeyForObj(c, &Job{JobID: jobID}),
  			TombstonesDelay: 30 * time.Minute,
  		},
  	}
  }
docs: // recentlyFinishedSet is a set with IDs of all recently finished invocations.
  //
  // This is an accumulator of IDs to remove from ActiveInvocations list next
  // time we run a triage for the corresponding Job.
  //
  // Invocation IDs are serialized with fmt.Sprintf("%d").