[tome]

I believe Zygote's approach is based on the backpropagator of Pearlmutter and Siskind. The back propagator approach seems much simpler to me, although the authors of this paper suggest that it requires non-local program transformations

> The implementation proposed by Pearlmutter and Siskind returns a pair of a value and a backpropagator ... Doing this correctly requires a non-local program transformation ... Further tweaks arerequired if a lambda uses variables from an outer scope ... In contrast to Pearlmutter and Siskind [2008], using delimited continuations enables reverse-mode AD with only local transformations. Any underlying non-local transformations are implicitly resolved by shift and reset.

I'll have to look more carefully to understand how the CPS version avoids non-local program transformations.