  The main idea of [4] was that structures built from periodic prime ideals
have better properties from the usual ones built from invariant ideals; but
unable to work with periodic ideals alone, we had to generalise further to a
somewhat ephemeral setting called virtual ideals. This text has two purposes.
It corrects an error in [4] discovered by Tom Scanlon's UCB seminar, recovering
all results for all virtual ideals. In addition, based on results in [3], we
describe a wide family of difference equations where virtual ideals reduce to
periodic ideals.
