NothingI didn't want to embarrass myself in public, so I looked at the whole owl:Thing vs. owl:Nothing again. Something I'd forgotten is that owl:Nothing cannot be instantiated. So it's OK if owl:Nothing seems to be a contradiction (I think), since it is only the instances of a class which create problems, and you can't have an instance of owl:Nothing.
It's after midnight. I'd better get to bed before my head hurts any more.