code: func (g *Graveyard) GetMaxIndexTxn(tx *memdb.Txn, prefix string) (uint64, error) {
  	stones, err := tx.Get("tombstones", "id_prefix", prefix)
  	if err != nil {
  		return 0, fmt.Errorf("failed querying tombstones: %s", err)
  	}
  
  	var lindex uint64
  	for stone := stones.Next(); stone != nil; stone = stones.Next() {
  		s := stone.(*Tombstone)
  		if s.Index > lindex {
  			lindex = s.Index
  		}
  	}
  	return lindex, nil
  }
docs: // GetMaxIndexTxn returns the highest index tombstone whose key matches the
  // given context, using a prefix match.