Function fair_margin_with 

pub fn fair_margin_with<R: Scalar>(
    swap: &BondTotalReturnSwap,
    view: &PricingView<'_, R>,
) -> Result<R, PricingError>

The funding margin that zeroes a total return swap, closed-form.

The margin sits on one leg and moves no cashflow’s date, so the package is exactly affine in it: funding(m) = funding(0) + m·A with A the funding leg’s annuity, and the fair margin is (return − funding(0)) / A — a division, not a solve.

Errors

PricingError::DegenerateAnnuity for a matured swap; otherwise whatever pricing the legs reports.