Function expint_double 

pub fn expint_double<P: Policy, E, V>(x: V, n: u32) -> V
where
    E: FloatElementWithBits + ExpIntConsts<1>,
    V: FloatVectorWithBits<Element = E> + SpecializedSpecialMath<E>,

$E_n(x)$ only, for a runtime order. See expint_double_primal.