pub fn mul_fp2_by_nonresidue(a: &Fp2E) -> Fp2E {
    // (c0 + c1 * u) * (9 + u) = (9 * c0 - c1) + (9c1 + c0) * u
    let f = a.double().double().double();
    let c0 = &f.value()[0] - &a.value()[1] + &a.value()[0]; // 9c0 - c1
    let c1 = &f.value()[1] + &a.value()[1] + &a.value()[0]; // 9c1 + c0
    Fp2E::new([c0, c1])
}
Computes the multiplication of an element of fp2 by the level two non-residue 9+u.
crates/math/src/elliptic_curve/short_weierstrass/curves/bn_254/field_extension.rs