Function cumprinc 

pub fn cumprinc<P, F, T>(
    rate: f64,
    periods: u32,
    present_value: P,
    future_value: F,
    start_period: u32,
    end_period: u32,
    timing: T,
) -> FinanceResult<f64>
where
    P: Into<f64> + Copy,
    F: Into<f64> + Copy,
    T: Into<PaymentTiming>,

Cumulative principal paid between two periods inclusive (Excel CUMPRINC).

start_period and end_period are 1-based; require start_period <= end_period.

Errors

Construction failures plus FinanceError::InvalidPeriod for bad ranges.