- Observe that in the model on the previous page, three clafers (`main_tracks`, `tracks_in`, `tracks_out`) have identical properties (children) and identical constraints.  This redundancy is irritating. Real **variability models need reuse mechanisms** as any other large models.  

- In Clafer, we use **inheritance** to reuse similar units of variability. Inheritance was not present in the original FODA feature models.  

- In the example, we extract an abstraction `trackLine`, which has to be configured to be either `single` or `dual`.  The `main_tracks`, `tracks_in` and `tracks_out` are extended to inherit from it, instead of repeating the same configuration tree. 

- (not shown here) Clafers which specialize other clafers via inheritance, can be **further extended**, by adding more nested   clafers.

```clafer
abstract xor trackLine
    single
    dual

abstract RegionalStationConf
    main_tracks :trackLine
    tracks_in  :trackLine
    tracks_out :trackLine
    parking_track ?

MyConfiguration : RegionalStationConf
```

This also illustrates why it might be useful to **mix class modeling** constructs **with feature modeling**.  Feature modeling benefits from the reuse methods of OO-modeling, while class modeling benefits from very concise notation of feature models.

**Task 3:**

- Use `ClaferConfigurator` again to quickly check whether we get the same instances (to properly check this, using this tool, you need to investigate all 16 instances).

- As previously remember to switch to the Alloy instance generation, then press 'Run' and only then 'Get instances' (we are working on a new release of tools, but in the release used in this wiki the choco installation has some problems).

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

- Close the configurator and move to the page ![next](/next.png) [Domain Constraints]().

<hr>
**Previous**
<hr>

1. [Commonality]()
2. [Variability]()