code: func LookupChunkSeries(ir IndexReader, tr TombstoneReader, ms ...labels.Matcher) (ChunkSeriesSet, error) {
  	if tr == nil {
  		tr = newMemTombstones()
  	}
  	p, err := PostingsForMatchers(ir, ms...)
  	if err != nil {
  		return nil, err
  	}
  	return &baseChunkSeries{
  		p:          p,
  		index:      ir,
  		tombstones: tr,
  	}, nil
  }
docs: // LookupChunkSeries retrieves all series for the given matchers and returns a ChunkSeriesSet
  // over them. It drops chunks based on tombstones in the given reader.