--- Updating the EPS. This should be an atomic operation.
--- Note the delicate 'seq' which forces the EPS before putting it in the
--- variable. Otherwise what happens is that we get
--- write eps_var (....(unsafeRead eps_var)....)
--- and if the .... is strict, that's obviously bottom. By forcing it beforehand
--- we make the unsafeRead happen before we update the variable.
-
+-- | Update the external package state. Returns the second result of the
+-- modifier function.
+--
+-- This is an atomic operation and forces evaluation of the modified EPS in
+-- order to avoid space leaks.