How to use the icontract._checkers.add_invariant_checks function in icontract

if not self.enabled:
            return cls

        assert self._contract is not None, "self._contract must be set if the contract was enabled."

        if not hasattr(cls, "__invariants__"):
            invariants = []  # type: List[Contract]
            setattr(cls, "__invariants__", invariants)
        else:
            invariants = getattr(cls, "__invariants__")
            assert isinstance(invariants, list), \
                "Expected invariants of class {} to be a list, but got: {}".format(cls, type(invariants))

        invariants.append(self._contract)

        icontract._checkers.add_invariant_checks(cls=cls)

        return cls

def __new__(mlcs, name, bases, namespace, **kwargs):  # type: ignore
            """Create a class with inherited preconditions, postconditions and invariants."""
            _dbc_decorate_namespace(bases, namespace)

            cls = super().__new__(mlcs, name, bases, namespace, **kwargs)  # type: ignore

            if hasattr(cls, "__invariants__"):
                icontract._checkers.add_invariant_checks(cls=cls)

            return cls