-

    fn witness(num: BigInt, a: BigInt, d: &BigInt, s: usize) -> Result {
        let mut x = (&a).pow_mod(d, &num);

        if x == one() || x == (&num - BigInt::one()) { return Result::Composite }

        for _ in 0..s {
            x = (&x * &x) % #
            if x == one() { return Result::Composite }
            if x == (&num - BigInt::one()) { return Result::PropablyPrime }
        }

        Result::Composite
    }