> they are almost never used in any real automatic differentiation system They're efficient enough for first-order derivatives. For example they are used in Ce

> they are almost never used in any real automatic differentiation system They're efficient enough for first-order derivatives. For example they are used in Ceres, Google's library for non-linear least-squares optimization https://ceres-solver.googlesource.com/ceres-solver/+/master/include/ceres/jet.h https://ceres-solver.googlesource.com/ceres-solver/+/master/...