One thing to keep in mind when you are playing the hidden labels game is that, apriori, people don't know the labels, so they don't really know how the sets are related. That is why the general intersecting circle model is used and why you must allow for empty regions. If you start with the other possible models--concentric circles or disjoint circles--you could end up not being able to represent the situation that develops as the labels become known. That is, you would be making unwarranted assumptions from the start.
Ron Ward/Western Washington U/Bellingham, WA 98225