Mixed-domain transformer scenario:

An attribute has been added to make something configuration. Attribute allows undefined, but has a default value opposite to the behavior of legacy servers.

In this case a configured value opposite to the default should be discarded. Undefined or a configured value equal to the default should not be discarded.

In this scenario, anything not discarded should be rejected, whether defined or not.

Task is to add a variant like RejectAttributeChecker.DEFINED and RejectAttributeChecker.UNDEFINED, but which rejects both.