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 invokeDurationDays() {
        FunctionTestUtil.assertResult(stringFunction.invoke(Duration.ofDays(9)), "P9D");
        FunctionTestUtil.assertResult(stringFunction.invoke(Duration.ofDays(-9)), "-P9D");
    }