This directory is a work-in-progress for Drake's forthcoming replacement of
Eigen::AutoDiffScalar<Eigen::VectorXd>.
The namespace ad { ... } is chosen for it's compact size in debugging traces,
but is not otherwise exposed to users.
Users should continue to #include <drake/common/autodiff.h> and refer to
drake::AutoDiffXd in their code.
Once we are confident with our replacement implementation, we will switch that alias to refer to this new code instead of Eigen's AutoDiffScalar.