A `reference clafer` is a clafer whose value is another clafer.

We can define the `opposite` traffic light as follows

```clafer
abstract TrafficLight
    opposite -> TrafficLight
    xor signal 
        stop 
            [ opposite.signal.stop ]
        proceed
            [ opposite.signal.proceed ]

EastWest : TrafficLight
    [ opposite = WestEast ]

WestEast  : TrafficLight
    [ opposite = EastWest ]
```

```{.clafer .config}
```

> Back to the [conclusion of Part III](two traffic lights#conclusion-of-part-iii)