Broken Pipes?

I have what I think is a reasonable use case for Pipes. This blog was started to talk about the kids, so that I didn't forget things about them as they were growing up. But then over time I've obviously started blogging about technical subjects as well, since that's very dear to me! (Blogging is evidently very egocentric at times!)

So my desire was to create a pipe that only contained posts about Jython-related topics, so that Jython developers wouldn't get all the noise about family. This would have seemed a fairly simple thing to do.

1. Take an XML document.
2. Apply an XPath filter to it (//item/category/term = 'jython').

Sadly, I haven't been able to get this to work, and posting to the Pipes support forum didn't elucidate any way of doing it. So I've fallen back to filtering based on the title of each post, which seems a bit crappy to me. Tag / Labels / Categories are meta-data about my post, and I should be able to use that meta-data. But this one can handle a post with only a single tag, but doesn't handle posts that have multiple categories. So that leaves me with the option to either always have certain text in the title of each post, or only tag posts with a single category, which kind of misses the point of categorising items. What am I missing?