@Test
public void invokeYearMonth() {

    FunctionTestUtil.assertResult(
            yamFunction.invoke(YearMonth.of(2017, 6), Year.of(2020)),
            ComparablePeriod.of(2, 7, 0));
    FunctionTestUtil.assertResult(
            yamFunction.invoke(YearMonth.of(2017, 6), Year.of(2014)),
            ComparablePeriod.of(-3, -5, 0));
}