Function require_annuity 

fn require_annuity<R: Scalar>(
    annuity: &R,
    entry_point: &'static str,
    product: &'static str,
) -> Result<(), PricingError>

Rejects a zero annuity before a par measure divides by it.

The as-of filter yields an exactly-empty sum for a matured trade, so the comparison is against zero exactly — no epsilon is invented.