Function risky_from 

fn risky_from<R: Scalar>(
    flows: &[(NaiveDate, R)],
    face_at: impl Fn(NaiveDate) -> f64,
    default_from: NaiveDate,
    maturity: NaiveDate,
    view: &PricingView<'_, R>,
    product: &'static str,
) -> Result<R, PricingError>

risky_on_view with the default walk starting at default_from — a loan or deposit not yet drawn cannot default on principal it has not lent.