  This is an addendum to the paper "Some models of spin coherence and
decoherence in storage rings" by one of the authors [1] in which spin diffusion
in simple electron storage rings is studied. In particular, we illustrate in a
compact way, a key implication in the Epilogue of [1], namely that the exact
formalism of [1] delivers a rate of depolarisation which can differ from that
obtained by the conventional treatments of spin diffusion which rely on the use
of the derivative $\partial \hat n/\partial\eta$ [2,3,4]. As a vehicle we
consider a ring with a Siberian Snake and electron polarisation in the plane of
the ring (Machine II in [1]). For this simple setup with its one-dimensional
spin motion, we avoid having to deal directly with the Bloch equation [5,6] for
the polarisation density. Our treatment, which is deliberately pedagogical,
shows that the use of $\partial \hat n/\partial\eta$ provides a very good
approximation to the rate of spin depolarisation in the model considered. But
it then shows that the exact rate of depolarisation can be obtained by
replacing $\partial \hat n/\partial\eta$ by another derivative as suggested in
the Epilogue of [1], while giving a heuristic justification for the new
derivative.
