code: func ClientCert(certFile, keyFile string) Option {
  	return func(o *Options) error {
  		cert, err := tls.LoadX509KeyPair(certFile, keyFile)
  		if err != nil {
  			return fmt.Errorf("nats: error loading client certificate: %v", err)
  		}
  		cert.Leaf, err = x509.ParseCertificate(cert.Certificate[0])
  		if err != nil {
  			return fmt.Errorf("nats: error parsing client certificate: %v", err)
  		}
  		if o.TLSConfig == nil {
  			o.TLSConfig = &tls.Config{MinVersion: tls.VersionTLS12}
  		}
  		o.TLSConfig.Certificates = []tls.Certificate{cert}
  		o.Secure = true
  		return nil
  	}
  }
docs: // ClientCert is a helper option to provide the client certificate from a file.
  // If Secure is not already set this will set it as well.