public void testPlus_Years() {
        Years test2 = Years.years(2);
        Years test3 = Years.years(3);
        Years result = test2.plus(test3);
        assertEquals(2, test2.getYears());
        assertEquals(3, test3.getYears());
        assertEquals(5, result.getYears());
        
        assertEquals(1, Years.ONE.plus(Years.ZERO).getYears());
        assertEquals(1, Years.ONE.plus((Years) null).getYears());
        
        try {
            Years.MAX_VALUE.plus(Years.ONE);
            fail();
        } catch (ArithmeticException ex) {
            // expected
        }
    }