Function solve_ladder 

fn solve_ladder(
    reference: NaiveDate,
    rungs: &[Rung],
    fixings: &FixingStore,
    fx: &FxRates,
    tolerance: Tolerance,
    form: Form<'_>,
    scheme: Interpolation,
) -> Result<Calibrated, CalibrationError>

The one ladder loop behind both forms.

The forms differ only in where the curve under solve sits in the view: as the discount (and, by fallback, the projection) when discount is None, or as the projection over a passive lift of discount otherwise.