Remove the (alleged) sources of partiality

E.g.

```
lib/Language/Ask/OddEven.hs:36:3: error: [-Wincomplete-patterns, -Werror=incomplete-patterns]
    Pattern match(es) are non-exhaustive
    In an equation for ‘mappend’:
        Patterns of type ‘Even b a’, ‘Even b a’ not matched:
            Stop Stop
            Stop (_ :-\ _)
```