Function observe_day 

fn observe_day<R: Scalar>(
    fixings: &FixingsView<'_, R>,
    index: &str,
    day: NaiveDate,
    as_of: NaiveDate,
) -> Result<Option<R>, CashflowError>

One overnight day under FixingsView::observe’s rule, a day after as_of answered here without the call: observe can only project it, and asking it day by day cost a third of a calibration. Both strip walks read through this, so the rule is stated once.