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.
Macros and other tools that need syntax privilege used
`(current-code-inspector)' at the module top-level to try to
capture the right code inspector at load time. It's more
consistent to instead use the enclosing module's declaration-time
inspector, and `var-ref->mod-decl-insp' provides that. The
new function works only on references to anonymous variables,
which limits access to the inspector.
The real function name is longer, of course.
