```clafer
abstract Lamp
    xor shape
        ball
        square
        diamond

TrafficLightSystem
    Lamps
        Red : Lamp
            [ no diamond ]
        Yellow : Lamp
            [ no square ]
        Green : Lamp
            [ ball ]
    allBalls ?
        [ all l : Lamp | l.shape.ball ]
    xor orientation
        vertical
            [ allBalls ]
        horizontal
            [ Red.shape.square && Yellow.shape.diamond ]    
```

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

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