  In this brief report I introduce a yet another class of geometries for which
semi-classical chronology protection theorems are of dubious physical
reliability. I consider a ``Roman ring'' of traversable wormholes, wherein a
number of wormholes are arranged in a ring in such a manner that no subset of
wormholes is near to chronology violation, though the combined system can be
arbitrarily close to chronology violation. I show that (with enough wormholes
in the ring) the gravitational vacuum polarization (the expectation value of
the quantum stress-energy tensor) can be made arbitrarily small. In particular
the back-reaction can be kept arbitrarily small all the way to the
``reliability horizon''---so that semi-classical quantum gravity becomes
unreliable before the gravitational back reaction becomes large.
