public FEELFnResult<String> invoke(@ParameterName("from") Object val) {
        if ( val == null ) {
            return FEELFnResult.ofResult( null );
        } else {
            return FEELFnResult.ofResult( TypeUtil.formatValue(val, false) );
        }
    }
@Test
    void invokePeriodYears() {
        FunctionTestUtil.assertResult(stringFunction.invoke(Period.ofYears(24)), "P24Y");
        FunctionTestUtil.assertResult(stringFunction.invoke(Period.ofYears(-24)), "-P24Y");
    }