code: func NewChunkReader(r *bgzf.Reader, chunks []bgzf.Chunk) (*ChunkReader, error) {
  	b := r.Blocked
  	r.Blocked = true
  	if len(chunks) != 0 {
  		err := r.Seek(chunks[0].Begin)
  		if err != nil {
  			return nil, err
  		}
  	}
  	return &ChunkReader{r: r, wasBlocked: b, chunks: chunks}, nil
  }
docs: // NewChunkReader returns a ChunkReader to read from r, limiting the reads to
  // the provided chunks. The provided bgzf.Reader will be put into Blocked mode.