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I discovered this today as well while reading the doctest documentation.
One thing that I never noticed before (and that doesn't seem to be reflected in the comments above) is that many of the code snippet rectangles in the doctest documentation have a small rectangle in the upper-right corner with the text ">>>".
If you click on one of these small rectangles, most of the text inside the code snippet rectangle disappears. I haven't confirmed this, but I suspect that it is the code snippets with a doctest directive that have this clickable corner.
Is this a feature? What is it for? :)