public FEELFnResult<String> invoke(@ParameterName("string") String string) {
        if ( string == null ) {
            return FEELFnResult.ofError(new InvalidParametersEvent(Severity.ERROR, "string", "cannot be null"));
        } else {
            return FEELFnResult.ofResult( string.toLowerCase() );
        }
    }
@Test
    void invokeUppercaseString() {
        FunctionTestUtil.assertResult(stringLowerCaseFunction.invoke("TESTSTRING"), "teststring");
    }