Well George, I ran into this issue a couple of weeks ago, and I knew it won't happen anytime soon, so I did it my own way.
In the "Order" I created another attribute called "LastLineNumber". Every time each order gets a new line item, this number gets incremented, gets assigned to Line Item number and then LastLineNumber also gets updated too.
And anytime I delete all the items, this attribute gets rest to zero.
Works great and each order has it's own sequence.