docs: protected String getOperatorSymbol(XAbstractFeatureCall call) {
  		if (call != null) {
  			final Resource res = call.eResource();
  			if (res instanceof StorageAwareResource) {
  				final boolean isLoadedFromStorage = ((StorageAwareResource) res).isLoadedFromStorage();
  				if (isLoadedFromStorage) {
  					final QualifiedName operator = getOperatorMapping().getOperator(
  							QualifiedName.create(call.getFeature().getSimpleName()));
  					return Objects.toString(operator);
  				}
  			}
  			return call.getConcreteSyntaxFeatureName();
  		}
  		return null;
  	}