@Test
public void txn_threaded_01() {

    Txn.exec(counter, TxnType.READ_PROMOTE, ()->{
        ThreadAction a = ThreadTxn.threadTxnWrite(counter, ()->{});
        a.run();
        // Blocks promotion.
        boolean b = counter.promote();
        assertFalse(b);
        assertEquals(ReadWrite.READ, counter.transactionMode());
    });
}