[rules-users] on update

Folks, I need a rule that will track changes to cost amount and do something with his old value. Like that : rule "on updated cost write to archive old value" when cost : Cost() old_cost : Cost(amount != cost.amount) // how can I specify the OLD cost object before it was updated??? then ... write to archive $old_cost.amount end Is it possible at all? Something like in database system triggers, when you have "on update" and then you have OLD value and NEW values available...? Thanks a lot! -Sergey ********************** ** LEGAL DISCLAIMER ** ********************** This E-mail message and any attachments may contain legally privileged, confidential or proprietary information. If you are not the intended recipient(s), or the employee or agent responsible for delivery of this message to the intended recipient(s), you are hereby notified that any dissemination, distribution or copying of this E-mail message is strictly prohibited. If you have received this message in error, please immediately notify the sender and delete this E-mail message from your computer.