  Objecting that Sommerfeld's quantum conditions refer to particular
coordinates, Einstein proposes a canonically invariant rule. But even if the
invariance is canonical, Einstein may have in mind a double
\emph{configuration} space invariance: with respect to loop deformations, and
to \emph{point} transformation -- all on a torus where features of the
Liouville-Arnol'd theorem already appear.
