Just memo W can use [Eigen3 + QD](https://eigen.tuxfamily.org/bz/show_bug.cgi?id=276), but we need to patch `QD` because * `QD` does not have `expm1`. * Their version of `+` has fairly large accuracy problems.
Just memo
W can use Eigen3 + QD, but we need to patch
QDbecauseQDdoes not haveexpm1.+has fairly large accuracy problems.