Implement checking for non-existence of keys:

You can now check that a key does *not* exist in the parameters,
and not just whether it exists.

    my $rules = {
        Person => [
            Office => [
                [ 1, { burglary_kit => undef } ]
            ]
        ]
    };

This checks the Person does *not* have a burglary kit when trying
to access the Office.