ChainRulesCore.jl
                                
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                        RFC: turn off thunks at 2nd order
Why do we want to do this?
This was part of https://github.com/FluxML/Zygote.jl/issues/1262#issuecomment-1193400476, tl;dr is that everything 2nd order in Zygote is broken. Besides problems with the Z-to-CR bridge, the creation of thunks doesn't seem to be differentiable, and ProjectTo also gave problems. But far from sorted out.