Document order. You can define it for nodes in a document, obviously, but what about nodes from different documents? Or nodes disconnected from the main document (in a document fragment, perhaps). Well, you can't. XPath makes it pretty easy to deal with multiple documents within the same expression, so, however rare, attempting to sort nodes from different documents has to do something sensible. And now it does. Hurrah.
A few little clean-ups, move it over into the main source tree, and it'll be time to release.