code: func tlsConfig(certFile, keyFile string) (*tls.Config, error) {
  	if certFile == "" {
  		certFile = defaultCertFile
  	}
  	if keyFile == "" {
  		keyFile = defaultKeyFile
  	}
  	cert, err := tls.LoadX509KeyPair(certFile, keyFile)
  	if err != nil {
  		return nil, err
  	}
  	return &tls.Config{Certificates: []tls.Certificate{cert}}, nil
  }
docs: // Load the certificate from cert and key files, will use the default certificate if the provided info are invalid.