Function erfcx_with 

pub fn erfcx_with<V, E, P, const N: usize>(x: V, l: E, a: &[E; N]) -> V
where
    E: FloatElement,
    V: FloatVector<Element = E> + SpecializedTranscendentalMath<E>,
    P: Policy,

erfcx by the N-term Weideman approximation on the imaginary axis.

N is a literal at every call site (the ladder in erfcx_internal instantiates it as one of 8/16/24/32/40), which is what lets the trip count and the coefficient loads fold.