code: func domainsForRateLimiting(names []string) ([]string, error) {
  	var domains []string
  	for _, name := range names {
  		domain, err := publicsuffix.Domain(name)
  		if err != nil {
  			// The only possible errors are:
  			// (1) publicsuffix.Domain is giving garbage values
  			// (2) the public suffix is the domain itself
  			// We assume 2 and do not include it in the result.
  			continue
  		}
  		domains = append(domains, domain)
  	}
  	return core.UniqueLowerNames(domains), nil
  }
docs: // domainsForRateLimiting transforms a list of FQDNs into a list of eTLD+1's
  // for the purpose of rate limiting. It also de-duplicates the output
  // domains. Exact public suffix matches are not included.