> Verb backout... Ann wrote about undo list.
> I hope this is not affected by isc_tpb_no_auto_undo.

AFAIU, undo log is always maintained by the engine.
isc_tpb_no_auto_undo tell how to perform ROLLBACK -
backout all job done with help of undo log and mark transaction
as commited in TIP or simple mark transaction as dead in TIP
and stuck OIT.

Regards,
Vlad