Function known_amounts 

fn known_amounts(
    known: El<'_, '_>,
    el: El<'_, '_>,
    n: usize,
    we_pay: bool,
    boundaries: &[NaiveDate],
    groups: &[(usize, usize)],
    payment_date: &dyn Fn(NaiveDate) -> NaiveDate,
) -> Result<StreamLeg, FpmlError>

A stream whose knownAmountSchedule states what each payment pays: one dated amount per payment, the amount in force at its period’s start.