Basically, Racklog (and all versions of schelog) implement ! by
causing the failure continuation of the entire relation being
returned. They did not also cause the unification caused by the
relation to be un-done.
However, it is not easy to separate un-doing the local changes because
the unification just returns a failure continuation too. I had to call
that fail continuation but use state to communicate to its target that
the next clause should not be visited.
I don't know if this is correct. My test suite contains a lot of cut
tests that still pass. Erik's test passes too. But I'm not confident
that this really works.
