Some very tough computational geometry problems I couldn't solve on my own, nor with the assistance of other AIs or my colleagues. Fable did them all first go. The most impressive built a custom optimizer with a ludicrous number of adaptive switches that absolutely crunched through an error surface with a bunch of nontrivial nullspaces and some wild curvature. That optimizer is of independent interest; it's not totally novel in theory, but the implementation is an impressive piece of engineering.