```prototype
/// Add the last two numbers of the input slice.
///
/// If the slice is not large enough, return `None`
/// If it is, return the computed value in a `Some`
pub fn add_last_two(v: &[f32]) -> Option<f32> {
    unimplemented!()
}
```

```example
fn main() {
    assert_eq!(add_last_two(&[]), None);
    assert_eq!(add_last_two(&[10.0]), None);
    assert_eq!(add_last_two(&[1.0, 2.0, 3.0]), Some(5.0));
}
```