code: func (s *Server) AppendCert(certFile, keyFile string) error {
  	if len(certFile) == 0 && len(keyFile) == 0 {
  		return errNoCertOrKeyProvided
  	}
  
  	cert, err := tls.LoadX509KeyPair(certFile, keyFile)
  	if err != nil {
  		return fmt.Errorf("cannot load TLS key pair from certFile=%q and keyFile=%q: %s", certFile, keyFile, err)
  	}
  
  	s.configTLS()
  
  	s.tlsConfig.Certificates = append(s.tlsConfig.Certificates, cert)
  	return nil
  }
docs: // AppendCert appends certificate and keyfile to TLS Configuration.
  //
  // This function allows programmer to handle multiple domains
  // in one server structure. See examples/multidomain