Swift3.0 support

```
        func convert<S: NumericType>(_ s: S, arr: [S], f: (S) -> S) -> [S] {
            switch arr.match {
            case let .some(h, t):   return [f(h) + s] as! [] + convert(h + s, arr:t, f: f)
            case .none:             return []
            }
        }
```

I dont know how to re-write this part in swift 3
