I have no doubt that there is a quick and easy way to do this in XSLT 2.

I have no doubt that there is a way to do this via some grouping
methodology that I haven't thought about.  But my brain is a bit
tired.

Yet, no one has responded to your email.  Since it's likely there a
quite a few people who won't see it for a couple of hours I'll tell
you how I would approach this right off the top of my head. But it
wouldn't be good.  Thankfully better was ways are already covered in
the FAQ http://www.dpawson.co.uk/xsl/sect2/N4486.html#d5151e1051 and
on Jenni's site (cited in the FAQ
http://www.jenitennison.com/xslt/grouping/muenchian.html.

Jon Gorman