Function optioned 

fn optioned<R: Scalar, C: CurveByIndex<R> + ?Sized>(
    payoff: &RatePayoff,
    rate: R,
    terms: &OptionTerms,
    curves: &C,
) -> Result<R, CashflowError>

A coupon rate rate = g·F + s still to fix, with payoff on it: each option an optionlet on the index at (K − s)/g, times |g|, a negative gearing swapping calls for puts; Bachelier or Black-76 by the surface’s convention, the vol read in the mode at terms.expiry and the tenor, its variance over terms.variance. A collar is r − caplet(cap) + floorlet(floor); a digital is its call spread (see Digital).