Function phi_internal 

pub fn phi_internal<V, E, P, const ADAPTIVE: bool>(
    z: V,
    n: u32,
    terms: usize,
) -> V
where
    E: FloatElement,
    V: FloatVector<Element = E> + SpecializedTranscendentalMath<E>,
    P: Policy,

The runtime-order twin of phi_internal_n: the same two arms with N as a value. The 1/N! prefactor and the per-term ratios are the same running products, so the two forms agree to the bit for the same terms.