'Run a setup method, framing the operation with a Base class
that will catch new subclasses to be established within
the "classes" registry.'

@classmethod
def _with_register_classes(cls, fn):

    cls_registry = cls.classes
    class FindFixture(type, ):
        def __init__(cls, classname, bases, dict_):
            cls_registry[classname] = cls
            return type.__init__(cls, classname, bases, dict_)
    class _Base(util.with_metaclass(FindFixture, object), ):
        pass
    class Basic(BasicEntity, _Base, ):
        pass
    class Comparable(ComparableEntity, _Base, ):
        pass
    cls.Basic = Basic
    cls.Comparable = Comparable
    fn()
