code: func (pb *Block) Tombstones() (TombstoneReader, error) {
  	if err := pb.startRead(); err != nil {
  		return nil, err
  	}
  	return blockTombstoneReader{TombstoneReader: pb.tombstones, b: pb}, nil
  }
docs: // Tombstones returns a new TombstoneReader against the block data.