The restriction on "returning to your point of origin via another RT fare" ticketing (gee, is that name sufficiently unambiguous yet?) is still pretty hard to draw a line on.

So assume that BOS-XXX and XXX-BOS RTs on the same airline are out. BOS-XXX and XXX-BOS on different airlines, we've discussed that too.

How about BOS-XXX with an XXX-PVD round trip in between? BOS and PVD aren't co-terminals, after all. Or MHT-XXX and XXX-PVD, even.

Or, say, you've been scheduled for a conference at XXX, and the company paid for your BOS-XXX ticket. Then the conference is rescheduled to only be half of the original scheduled time, so you buy an XXX-BOS ticket so you can come home and do laundry....