pub fn mul_fp2_by_nonresidue(a: &Fp2E) -> Fp2E {
    // (c0 + c1 * u) * (1 + u) = (c0 - c1) + (c1 + c0) * u
    let c0 = &a.value()[0] - &a.value()[1]; // c0 - c1
    let c1 = &a.value()[0] + &a.value()[1]; // c1 + 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/bls12_381/field_extension.rs