fn value_nans_last<T: Float>(a: &T, b: &T) -> Ordering {
      match (a, b) {
        (x, y) if x.is_nan() && y.is_nan() => Ordering::Equal,
        (x, _) if x.is_nan() => Ordering::Greater,
        (_, y) if y.is_nan() => Ordering::Less,
        (_, _) => a.partial_cmp(b).unwrap()
      }
    }

    xs.sort_by(value_nans_last);