code: func detectConfigChange(conf jsonconfig.Obj) error {
  	oldHTTPSKey, oldHTTPSCert := conf.OptionalString("HTTPSKeyFile", ""), conf.OptionalString("HTTPSCertFile", "")
  	if oldHTTPSKey != "" || oldHTTPSCert != "" {
  		return fmt.Errorf("config keys %q and %q have respectively been renamed to %q and %q, please fix your server config",
  			"HTTPSKeyFile", "HTTPSCertFile", "httpsKey", "httpsCert")
  	}
  	return nil
  }
docs: // detectConfigChange returns an informative error if conf contains obsolete keys.