fn example_verify_x() {
    let mut rng = ChaCha20Rng::from_seed([1;32]);
    let x: Fr = Fr::rand(&mut rng);
    let pk: K = K::rand(&mut rng);
    let d: K = pk.mul(x).into();
    // generate DLEQ proof
    let proof = prepare_proof(x, d, pk);
    // check validity
    let is_valid = verify_proof(pk, d, proof);
    // should be valid
    assert!(is_valid);
}