Function phi1 

pub(crate) fn phi1<R: Scalar>(a: R, x: f64) -> R

φ(a, x) = (1 − e^{−a·x}) / a, the duration factor over a span x, summed as its series x·Σ (−ax)^k/(k+1)! where |a·x| < 0.5 so it tends to x as a → 0 without cancellation. In the mode.