code: func NewBlockQuerier(b BlockReader, mint, maxt int64) (Querier, error) {
  	indexr, err := b.Index()
  	if err != nil {
  		return nil, errors.Wrapf(err, "open index reader")
  	}
  	chunkr, err := b.Chunks()
  	if err != nil {
  		indexr.Close()
  		return nil, errors.Wrapf(err, "open chunk reader")
  	}
  	tombsr, err := b.Tombstones()
  	if err != nil {
  		indexr.Close()
  		chunkr.Close()
  		return nil, errors.Wrapf(err, "open tombstone reader")
  	}
  	return &blockQuerier{
  		mint:       mint,
  		maxt:       maxt,
  		index:      indexr,
  		chunks:     chunkr,
  		tombstones: tombsr,
  	}, nil
  }
docs: // NewBlockQuerier returns a querier against the reader.