@Override
    public int hashCode() {
        return timestamp.hashCode();
    }
@Test
    public final void testHashCode() {
        Timestamped<String> a = new Timestamped<>("a", TS_1_1);
        Timestamped<String> b = new Timestamped<>("b", TS_1_1);
        assertTrue("value does not impact hashCode",
                a.hashCode() == b.hashCode());
    }