func prime_factors(n) {
    return [] if (n < 1)
    gather {
        while (!(n & 1)) {
            n >>= 1
            take(2)
        }
        var p = 3
        while ((n > 1) && (p*p <= n)) {
            while (n %% p) {
                n //= p
                take(p)
            }
            p += 2
        }
        take(n) if (n > 1)
    }
}