Indentation/formatting, should be either
```rust
let (hi, lo) = twosum(partials.as_slice()[0] * twopow,
                      0.5 * partials.as_slice()[plen - 1]);
```

or, preferably:

```rust
let (hi, lo) = twosum(partials.get(0) * twopow, 0.5 * partials.get(plen - 1));
```