如何使用XPath选择最后N个元素？

I support a web site which generates content XML that is then translated into web pages using XSLT. I have been asked to create a new stylesheet which will transform the output of the "archive" page into Atom for syndication. The problem I'm running into is that the archive page contains a rather large number of items — 142 and counting — and the feed should never have more than thirty items.I support a web site which generates content XM