docs: @Override
     public void begin() throws NotSupportedException, SystemException {
        Transaction currentTx;
        if ((currentTx = getTransaction()) != null)
           throw new NotSupportedException(Thread.currentThread() +
                 " is already associated with a transaction (" + currentTx + ")");
        DummyTransaction tx = new DummyTransaction(this);
        setTransaction(tx);
     }