float angle_rad = acos(q0) * 2;
    float x = q1 / sin(angle_rad/2);
    float y = q2 / sin(angle_rad/2);
    float z = q3 / sin(angle_rad/2);

void rotate(Quaternion *q) {
            // cprogramming.com/tutorial/3d/quaternions.html
            // euclideanspace.com/maths/algebra/realNormedAlgebra/quaternions/…
            // content.gpwiki.org/index.php/…
            // ^ or: webcache.googleusercontent.com/…

        void rotate(Quaternion *q) {
            Quaternion p(0, x, y, z);

            // quaternion multiplication: q * p, stored back in p
            p = q -> getProduct(p);

            // quaternion multiplication: p * conj(q), stored back in p
            p = p.getProduct(q -> getConjugate());

            // p quaternion is now [0, x', y', z']
            x = p.x;
            y = p.y;
            z = p.z;
        }