- It is common in domain modeling and domain engineering to focus on **narrow domains**.  This helps to avoid **scope creep** and exploit **commonality**.  Managing **variability* is the price we have to pay for being able to support more than one product, but we want to keep the variability possible low (as a cost).  

- In this tutorial, we will concentrate our model on small regional stations. We scope our model to:
    + allow single or dual main tracks, 
    + one or two input tracks, 
    + and one or two output tracks.  

- Finally, we observe that some stations (and only some) also have a side track, also known as  a parking track, were trains can be stopped for longer periods.

Let's reflect these changes in our feature model:

```clafer
abstract RegionalStationConf
    xor main_tracks
        single 0..1
        dual 0..1
    1..1 tracks_in
        single ? // shortcut for 0..1
        dual ?
    xor tracks_out
        single // '?' is default for grouped clafers
        dual   // so we omit it.
    parking_track ? // this clafer is not grouped, so '?' cannot be omitted


MyConfiguration : RegionalStationConf
```


A few observations about this model

- Recall that the symbol `?` is a handy notation for `0..1` cardinality and we will use it from now onwards.  This cardinality represents **optional** features, so this is one way we introduce **variability** in the model.

- Cardinality constraints that are written to the left of the clafer name restrict the cardinality of the set of all children.  We call such a cardinality constraint a **group cardinality** (as opposed to clafer cardinality)

- `xor` is a shortcut for `1..1` again, but traditionally used for group cardinality describing a choice of an **alternative**. It means that only one of the children can be selected at a time.  Think of this group as 'Radio Buttons'.

- other useful group cardinalities include `mux` (which stands for `0..1`), and `or` (which stands for `1..*`).  The former means that at most one sub-clafer can be instantiated, but perhaps none. This is a classic mutual exclusion requirement (**an exclusion group**).  The latter, defining so called **or-groups**, means that at least one sub-clafer must be instantiated.  This captures a common pattern of a required subsystem, that has a number of variations, which can coexist; for instance, different compression algorithms or different authentication protocols in software applications.  

> More information: [Tutorial: Group Cardinalities](https://github.com/gsdlab/clafer/wiki/Group-Cardinality)

- If the group cardinality constraint is anything else than `0..*` (which is the default one, corresponding to no restriction), then the default cardinality for each of the children is `?` (otherwise `1` as explained in the previous page).  This is natural, as mandatory features do not make sense in groups with restricted cardinality -- they would immediately produce an inconsistency in the model.

- Clafers can be **arbitrarily nested**. Thus our mandatory clafers (first level of nesting) still are mandatory, but their subclafers are optional.

> More information: [Tutorial: Cardinality](https://github.com/gsdlab/clafer/wiki/Cardinality)

How many `model instances` are possible now? 

Let's use another tool, `ClaferConfigurator`, to generate all instances.

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

**Task 2:**

- Press the button to open the model from this page in the configurator and press the button `Run` in the `Instance Generator` window -- it will try to generate 10 instances. Make sure that you are using an Alloy based generator, not the choco and not the Z3 based.

- The green tick ![green tick](/tick.png) indicates the presence of a clafer in a given instance, the red crossed circle ![red crossed circle](/no.png) its absence. 
All mandatory clafers are also present in every instance as indicated by gray checked checkboxes ![always checked checkbox](/bool_always_checked.png) in the left column.

- Note that, in general, making a clafer mandatory does not guarantee that it will be present in all instances, since it can be a child of an optional clafer. 

- Close the configurator

For completeness we include the same model in the popular FODA [FODA] notation:

![Small train stations, variability in the FODA feature modeling syntax](/fm.png)


Clafer also has a simple visualization tool translating models to the VSpec notation of Common Variability Language (CVL).  This is very similar to FODA feature models:

```{.clafer .cvlGraph}
```

- Move to the page ![next](/next.png) [Do not Repeat Yourself]().

<hr>
**Previous**
<hr>

1. [Commonality]().