Redundant parentheses in case split

Before case splitting:
```
No contra => case isElem value xs of
                  case_val => ?isElem_rhs_3
```

After it:
```
No contra => case isElem value xs of
                  (Yes prf) => ?isElem_rhs_1
                  (No contra) => ?isElem_rhs_2
```

Couldn't these parentheses be omitted automatically?
