@staticmethod
def _create_state(new_class, name, value) ->None:
    """Create a state hidden in new class."""
    if new_class._states is None:
        new_class._states = {}
    states = new_class._states
    if name in states:
        raise RuntimeError('Duplicated states are not allowed {!r} {!r}'.
            format(new_class, name))
    states[name] = value
