@classmethod
def register_class_callback(cls, cbk, *args, **kwargs):
    """
        Register a class level callback `cbk` to be applied when reading a frame.

        The callback must receive a `System` instance as input an
        return the modified `System` instance as output.

        Class level callbacks are applied to all instances of a class.
        """
    if cls.class_callbacks is None:
        cls.class_callbacks = []
    if (cbk, args, kwargs) not in cls.class_callbacks:
        cls.class_callbacks.append((cbk, args, kwargs))
