  In this expository paper, we give a complete proof of van den Essen's theorem
that the de Rham cohomology spaces of a holonomic D-module are
finite-dimensional in the case of a formal power series ring over a field of
characteristic zero. This proof requires results from at least five of van den
Essen's papers as well as his unpublished thesis, and until now has not been
available in a self-contained document.
