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Part I
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We will be modeling a domain of small regional train stations, restricting our attention to track layouts. Let's first focus on the commonality among the different train stations.  Each regional train station will have some **main tracks**, and some **incoming** and **outgoing** tracks, connecting it to the main train systems.

```clafer
abstract RegionalStationConf
    main_tracks 1..1 // a cardinality constraint following a clafer restricts the number of its instances
    tracks_in 1         // 1..1 is written 1 in short
    tracks_out          // 1 is default, so it can be omitted
```

In a Clafer model, every line declares a new `clafer`, which represents a concept or a property. 
The clafers are nested using indentation (`red` is a child clafer of `Lamps`). 
Clafers are followed by `clafer cardinality`, which indicates the allowed number of their instances: 
in general, the cardinality is an interval `n..m`, where `m` can be `*` to indicate infinity.
A clafer instance can exist only when its parent clafer's instance exists (e.g., instance of `red` cannot exist without an instance of `Lamps`).
In our model, we use `1..1` and a shorthand `1` to indicate `mandatory` clafers, that is, clafers which must exist in every instance of the model as long as their parent exists. 
For `green`, we do not specify any cardinality since mandatory is the default cardinality and therefore it can be omitted.

What are the `model instances`? In our case, we can interpret our `model` as a description of variability across all jurisdictions, whereas the instances of our model will correspond to a particular variant of the system for a given jurisdiction. Since all clafers in our model are mandatory, there is only a single instance of our model (i.e., there is actually no variability yet). 

Let's use the tool `ClaferIDE` to generate the instance.

```{.clafer .ide}
```

Press the button to open the model from this page in the IDE and press the button `Run` in the `Instance Generator` window. Then press the button `Next` to generate another instance and see what happens.

Close the IDE and move to the page ![next](/next.png) [adding variability]().