This hook wraps ``normalize_slot``, and performs clean-ups which
        require access to the object the slot is in as well as the value.

def normalize_object_slot(self, value=_nothing, prop=None, obj=None):
        """This hook wraps ``normalize_slot``, and performs clean-ups which
        require access to the object the slot is in as well as the value.
        """
        if value is not _nothing and hasattr(prop, "compare_as"):
            method, nargs = getattr(prop, "compare_as_info", (False, 1))
            args = []
            if method:
                args.append(obj)
            if nargs:
                args.append(value)
            value = prop.compare_as(*args)
        return self.normalize_slot(value, prop)