code: func sanitizeDomain(domains []string) []string {
  	var sanitizedDomains []string
  	for _, domain := range domains {
  		sanitizedDomain, err := idna.ToASCII(domain)
  		if err != nil {
  			log.Infof("skip domain %q: unable to sanitize (punnycode): %v", domain, err)
  		} else {
  			sanitizedDomains = append(sanitizedDomains, sanitizedDomain)
  		}
  	}
  	return sanitizedDomains
  }
docs: // https://tools.ietf.org/html/draft-ietf-acme-acme-16#section-7.1.4
  // The domain name MUST be encoded
  //   in the form in which it would appear in a certificate.  That is, it
  //   MUST be encoded according to the rules in Section 7 of [RFC5280].
  //
  // https://tools.ietf.org/html/rfc5280#section-7