def order(self, mechanism, purview):
        """Order the mechanism and purview in time.

        If the direction is ``CAUSE``, then the purview is at |t-1| and the
        mechanism is at time |t|. If the direction is ``EFFECT``, then the
        mechanism is at time |t| and the purview is at |t+1|.
        """
        if self is Direction.CAUSE:
            return purview, mechanism
        elif self is Direction.EFFECT:
            return mechanism, purview

        from . import validate
        return validate.direction(self)
Order the mechanism and purview in time.

        If the direction is ``CAUSE``, then the purview is at |t-1| and the
        mechanism is at time |t|. If the direction is ``EFFECT``, then the
        mechanism is at time |t| and the purview is at |t+1|.