Function cubic_2_points 

pub fn cubic_2_points(
    a: &Ratio,
    b: &Ratio,
    v1: &Ratio,
    v2: &Ratio,
    v3: &Ratio,
    v4: &Ratio,
) -> Polynomial

f(a) = v1, f(b) = v2, f’(a) = v3, f’(b) = v4
It ignores v2 and v4 if a == b.