Function hull_white_basis 

fn hull_white_basis<R: Scalar>(
    underlying: &SwaptionUnderlying,
    start: NaiveDate,
    entered: &[(NaiveDate, f64)],
    view: &PricingView<'_, R>,
) -> Result<R, PricingError>

The deterministic basis spread a Hull-White swaption strikes on, entering on start the fixed periods entered as (payment date, accrual): s = (P(S) − P(tₙ) − FL)/A per unit notional, so that the discount par leg the one-factor model values, P(S) − P(tₙ), less (K + s)·A is the delivered swap’s own value FL − K·A. FL is the present value of the floating coupons whose accrual starts on or after S, off the index’s projection curve — the payment lag, a projection curve other than the discount curve and a float schedule other than the fixed one are all in it. In the mode, so the basis carries its curve risk.