This would be a million times clearer if you save the `...[len -1]` and `...[len - 2]` to temporaries.

```rust
if len >= 2 {
     let (a, b) = (partials.as_slice()[len - 1], partials.as_slice()[len - 2]);
     if samesign(a, b) && (total_so_far + 2.0 * a - total_so_far) == 2.0 * a { ... }
}