code: func (g *Graveyard) InsertTxn(tx *memdb.Txn, key string, idx uint64) error {
  	// Insert the tombstone.
  	stone := &Tombstone{Key: key, Index: idx}
  	if err := tx.Insert("tombstones", stone); err != nil {
  		return fmt.Errorf("failed inserting tombstone: %s", err)
  	}
  
  	if err := tx.Insert("index", &IndexEntry{"tombstones", idx}); err != nil {
  		return fmt.Errorf("failed updating index: %s", err)
  	}
  
  	// If GC is configured, then we hint that this index requires reaping.
  	if g.gc != nil {
  		tx.Defer(func() { g.gc.Hint(idx) })
  	}
  	return nil
  }
docs: // InsertTxn adds a new tombstone.