One more optimization - Invoice - document reference

["David RR Webber \(XML\)" <david@drrw.info>]

OK this one is small - but there is a lot of them - and every bit (or
byte!) counts I guess - 

We have document references - this could easily be a repeating line with
object type - invoice, credit note, order, etc, etc

DW

"The way to be is to do" - Confucius (551-472 B.C.)