@Test
public void txn_nested_07() {

    Txn.exec(counter, TxnType.READ_COMMITTED_PROMOTE, ()->{
        boolean b = counter.promote();
        assertTrue(b);
        // Must the same type to nest Txn.
        Txn.exec(counter, TxnType.READ_COMMITTED_PROMOTE, ()->{});
    });
}