Function restate 

fn restate(
    vol: f64,
    from: VolConvention,
    spec: &SabrSpec,
    forward: f64,
    strike: f64,
    t: f64,
) -> Option<(f64, f64)>

A quote in convention from restated in the spec’s, by repricing a call at the slice forward — the lognormal side read at the spec’s shift — and the restatement’s slope in the quote: the ratio of the two vegas, since the price is held.