[Zope] Help (emergency) How to Undo last ZODB transaction when Zope is down

Hi, I'm in panic... I made an error when changing a site access rule and this caused ZMI to be not available. So I can't undo tat through the ZMI ! How to undo the latest transactions from command line ? The bbb.py provided with Zope plays only with ZODB2 files (in fact it's useless till it's not upgraded) TIA (it's an emergency) --Gerd