How to identify rollback in progress?

Anyone know of a quick way to identify any/all transactions currently being rolled back? I know I could look for decreasing used_ublk in v$transaction, or increasing undo statistics in v$sesstat, but is there anywhere that a transaction is simply flagged as currently being rolled back, so you can quickly identify it w/o having to check for deltas?  

It seems v$transaction.status would be the sensible place to put such a flag, but the only description of this column in the documentation is "status" - wow, that's really helpful. From what I've been able to see, the only values for status are ACTIVE and INACTIVE, but maybe I just didn't run my test rollback long enough to capture it.  

Thanks,
Brandon

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