code: func (r *Reader) Symbols() (map[string]struct{}, error) {
  	res := make(map[string]struct{}, len(r.symbolsV1)+len(r.symbolsV2))
  
  	for _, s := range r.symbolsV1 {
  		res[s] = struct{}{}
  	}
  	for _, s := range r.symbolsV2 {
  		res[s] = struct{}{}
  	}
  	return res, nil
  }
docs: // Symbols returns a set of symbols that exist within the index.