There wasn't one. Last month I failed to think. It's a strain, you know.
I should remember that the brain needs fuel. There's an impressive guy called Ori Hofmeckler (artist and athlete and a former member of the Israeli Special Forces) who sells a diet plan called The Warrior's Diet. Unlike other food gurus, this guy is the best advert for his own ideas: he's in his fifties, and (assuming the photos aren't doctored -- I haven't met him) he looks like Bruce Lee but with better abs.
Anyway, he recommends sometimes alternating between high-carb and high-protein days. I tend to follow a tailored version -- the Enhanced Warrior's Diet With Added Chocolate -- and I've been experimenting with alternating high- and low-chocolate days. Well, my brother Colm did warn me of the dangers of chocolate-reserve depletion...
I'll try to remember to keep eating enough chocolate to think with.