// in-place modification
int basis(vector<ll>& a) {
    int n = a.size();
    int has = 0;
    for (int i = 63; ~i and has < n; --i) {
        for (int j = has; j < n; ++j) {
            if (a[j] & (ll(1) << i)) {
                swap(a[j], a[has]);
                break;
            }
        }
        if (not (a[has] & (ll(1) << i))) continue;
        for (int j = 0; j < n; ++j) {
            if (j == has) continue;
            if (a[j] & (ll(1) << i)) {
                a[j] ^= a[has];
            }
        }
        ++has;
    }
    return has;
}