public static <U> MapValue<U> tombstone(Timestamp timestamp) {
        return new MapValue<>(null, timestamp, System.currentTimeMillis());
    }
@Test
    public void testTombstone() {
        Timestamp ts1 = new LogicalTimestamp(9);
        MapValue<String> mv = MapValue.tombstone(ts1);
        assertTrue(mv.isTombstone());
        assertFalse(mv.isAlive());
        assertNull(mv.get());
        assertEquals(ts1, mv.timestamp());
    }