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A Fundamental Theorem of Calculus for Lebesgue Integration, J. J. Koliha begins with the passage Lebesgue proved a number of remarkable results on the relation between integration and differentiation....

(To clarify, my interest is mainly lightface, that is, $\Pi^1_2$ instead of $\bf \Pi^1_2$, although it doesn't particularly matter.) This is just an idle curiosity. In logic, I find myself frequently ...