code: func TopicFromCert(cert tls.Certificate) string {
  	commonName := cert.Leaf.Subject.CommonName
  
  	var topic string
  	// Apple Push Services: {bundle}
  	// Apple Development IOS Push Services: {bundle}
  	n := strings.Index(commonName, ":")
  	if n != -1 {
  		topic = strings.TrimSpace(commonName[n+1:])
  	}
  	return topic
  }
docs: // TopicFromCert extracts topic from a certificate's common name.