Function exercise_steps 

fn exercise_steps(
    live: Vec<&Callability>,
    american: bool,
    grid: &TimeGrid,
    cut: NaiveDate,
    pays_on: impl Fn(NaiveDate) -> NaiveDate,
    strike: impl Fn(&Callability, NaiveDate) -> f64,
) -> Result<Vec<(usize, f64, CallKind)>, PricingError>

The (step, strike, kind) exercises of live entries after cut: each on its date rolled by pays_on (Bermudan), or under american on every slice from its date to the slice before the next entry’s, the last on its date alone. strike(entry, on) is the redemption on on, in currency.