Function expint_double_n 

pub fn expint_double_n<P: Policy, E, V, const N: usize>(x: V) -> V
where
    E: FloatElementWithBits + ExpIntConsts<N>,
    V: FloatVectorWithBits<Element = E> + SpecializedSpecialMath<E>,

$E_N(x)$ only. See expint_double_primal_n for the shape of the computation.