Tactics' agda-split can destroy matches

data ADT = One | Two Int | Three | Four Bool ADT | Five

```haskell
case_split :: ADT -> Int
case_split One = _
case_split (Two i) = _
case_split Three = _
case_split (Four b a) = _  -- here
case_split Five = _
```

Splitting on either `b` or `a` at `-- here` deletes the entire `case_split (Four b a)` match.

