In the Invoicing module I'd like to create Vendor bills which are individually sequenced by vendor. So now vendor bills look something like BILL2016xxxx where xxxx is the sequence number. The sequence number iterates by 0001 with each vendor bill. I want this iteration to happen separately by vendor, not globally over any vendor bill. If Alan gets BILL20160001 the next bill he gets should be BILL20160002 regardless of how many other vendor got bills in between these two events.