---
toc: false
...

Part II
=======

After finishing the initial domain analysis, we can move to considering how do these variations impact the control software of a traffic light. 
Let's first extract the features relevant to the control software from the [last version of our domain model](/country specific features).
We can remove the jurisdictions, constraints, and physical properties `mountLocation` and `orientation`, since they do not impact the control software in any way. 
We also remove `red`, `yellow`, and `green` because they are mandatory and `countdownTimer` as it won't be considered in this part.

```
TrafficLightSystem
    Lamps
        LeftGreenArrow ?
        RightGreenArrow ?
    redAndYellowToGreen ?
    allWayFlashingRedAsStop ?
    allWayYellow ?
    or advancedOrExtendedLeft ?
        greenFlashing
        leftArrow
```

Consider the above simplified model as the **`interface`** between the domain model and the control software.
The presence or absence of these features will be decided at the domain level but our control software must handle all possible decisions. 
This interface model describes all variability from the control software point of view.

From our domain analysis, we learned that all kinds of lamps, including the arrow lamps, can be in three states: `on`, `off`, and `flashing`. 
Let's focus first on a single traffic light. 
The images below show different traffic light configurations with all lamps turned on

<table>
<tr>
<td>

![a) main lamps only](allLampsOnMainLampsOnly.png)

</td>
<td>

![b) main and left arrow only](allLampsOnLeftArrowOnly.png)

</td>
<td>

![c) main and right arrow only](allLampsOnRightArrowOnly.png)

</td>
<td>

![d) all lamps, including both arrows](allLampsOn.png)

</td>
</tr>
<caption>Table 1: Allowed configurations of lamps of a traffic light</caption>
</table>

We can model the states of such a traffic light as follows:

```clafer
TrafficLight
    xor red 
        on
        off
        flashing
    xor yellow
        on
        off
        flashing
    xor green
        on
        off
        flashing
    xor leftGreenArrow ?
        on
        off
        flashing
    xor rightGreenArrow ?
        on
        off
        flashing
```

We can see all possible combinations of states in the configurator.

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

Given so much repetition in the model, it would be nice to introduce an abstraction of a lamp.
In Clafer, we can use inheritance for that purpose.
Let's pull up the ![next](/next.png) [lamp abstraction]().