@classmethod
def register_validator(cls, validator, group):
    """
        Registers another order validator in a validation group

        A validator is a callable accepting an order (and only an order).

        There are several types of order validators:

        - Base validators are always called
        - Cart validators: Need to validate for a valid cart
        - Checkout validators: Need to validate in the checkout process
        """
    cls.VALIDATORS.setdefault(group, []).append(validator)
