public Digest digest() {
        return new Digest(timestamp, isTombstone());
    }
@Test
    public void testDigest() {
        Timestamp ts = new LogicalTimestamp(10);
        MapValue<String> mv = new MapValue<>("foo", ts);
        Digest actual = mv.digest();
        Digest expected = new MapValue.Digest(ts, false);
        assertEquals(actual, expected);
    }