[abecedarius]

There's a more leisurely blog-post-style explanation of this complex-variable trick at https://codewords.recurse.com/issues/four/hack-the-derivativ...

[improbable22]

That's neat. It's perhaps worth mentioning that this trick is only needed when your computer understands complex numbers (1,i) but not dual numbers (1,ε), where i^2 = -1 but ε^2 = 0.