// we use a single txn op for all acks and update this op
// as new acks come in. If this is the first ack in the txn
// we will need to create and enlist the op.
if (_ackOp == null)
{
_ackOp = new TxAck(unacknowledgedMessageMap);
_txnBuffer.enlist(_ackOp);
}
// update the op to include this ack request
if (multiple && (deliveryTag == 0))
{