code: func fetchCertsJSON(c context.Context, url string) (*PublicCertificates, error) {
  	keysAndCerts := map[string]string{}
  	req := internal.Request{
  		Method: "GET",
  		URL:    url,
  		Out:    &keysAndCerts,
  	}
  	if err := req.Do(c); err != nil {
  		return nil, err
  	}
  
  	// Sort by key for reproducibility of return values.
  	keys := make([]string, 0, len(keysAndCerts))
  	for key := range keysAndCerts {
  		keys = append(keys, key)
  	}
  	sort.Strings(keys)
  
  	// Convert to PublicCertificates struct.
  	certs := &PublicCertificates{}
  	for _, key := range keys {
  		certs.Certificates = append(certs.Certificates, Certificate{
  			KeyName:            key,
  			X509CertificatePEM: keysAndCerts[key],
  		})
  	}
  	return certs, nil
  }
docs: // fetchCertsJSON loads certificates from a JSON dict "key id => x509 PEM cert".
  //
  // This is the format served by Google certificate endpoints.