code: func indexRRs(rrs []dns.RR, index map[string]dns.RR) {
  	for _, rr := range rrs {
  		name := strings.ToLower(rr.Header().Name)
  		if _, ok := index[name]; !ok {
  			index[name] = rr
  		}
  	}
  }
docs: // indexRRs populates a map which indexes a given list of RRs by name. NOTE that
  // the names are all squashed to lower case so we can perform case-insensitive
  // lookups; the RRs are not modified.