code: func (p *PopOp) makeTombstonesEntity() *tombstonesEntity {
  	p.entity.Tombstones = p.entity.Tombstones[:0]
  	for id, ts := range p.tombs {
  		p.entity.Tombstones = append(p.entity.Tombstones, struct {
  			ID         string
  			Tombstoned time.Time
  		}{id, ts})
  	}
  	return p.entity
  }
docs: // makeTombstonesEntity is used internally by FinishPop.