- Adding more constraints, will guide the search for interesting solutions further.

- Recall the feature model and meta-model:

```clafer
abstract xor trackLine
    single
    dual

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

abstract Track
    incident -> Track 0..3
        [ parent in this.incident ]
    [ no (this & incident) ]

abstract SimpleTrack :Track
    [ #incident = 2 ]

abstract Junction: Track
    [ #incident = 3 ]

abstract Line :Track
    [ one incident ]

abstract TrackBarrier :Track
    [ one incident ]
```

- We extend the Station clafer with a few more properties:

```clafer
abstract Station
    T :SimpleTrack 1..3
        [lone (incident & I) ] // a track should not connect more than one inputs
        [lone (incident & O) ] // (outputs)
    J :Junction 0..4
    I :Line 1..2
    O :Line 0..2

    B :TrackBarrier 0..2

    // a station must be self contained 

    [ T.incident in T ++ J ++ I ++ O ++ B ]
    [ J.incident in T ++ J ++ I ++ O ]
    [ I.incident in T ++ J ++ I ++ O ]
    [ O.incident in T ++ J ++ I ++ O ]
    [ B.incident in T ]


```



- Let's instantiate a station. 

```clafer
MyStation :Station
```

- Investigate some legal stations in Clafer IDE. Stop at the first instance that is 'weird' (it should not take more than three instances normally):

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

It looks like we need to ![next](/next.png) [Add more solution space constraints]()

<hr>
**Previous**
<hr>

Part I

1. [Commonality]()
2. [Variability]()
3. [Do not Repeat Yourself]()
4. [Domain Constraints]()

Part II

5. [Station Building Blocks]()

Part III

6. [Base Model]()