Now I'm not able to reproduce this bug. The only thing I've changed recently is moving ItemSet down to a lower package (from wpmcn to wpmcn.structure), but I'd be surprised if that was the problem.
I'll have to see if I can repro with an older version of the code. I've implemented ItemSet differently than you've implemented your custom writable. See https://gist.github.com/1214627. I don't think this is the issue, though.
