_ operator + (const _ &rhs) const {
        _ ret;
        unsigned p[32]{}, f[32]{};
        REP(i,0,31) p[i] = a[i];
        PER(i,0,31) {
            auto x = rhs.a[i], y = 1u<<i;
            PER(j,0,31) if (x>>j&1) {
                if (p[j]) x^=p[j],y^=f[j];
                else {
                    p[j]=x,f[j]=y;
                    break;
                }
            }
            if (x==0) {
                unsigned sum = 0;
                REP(j,0,31) if (y>>j&1) sum ^= rhs.a[j];
                ret.ins(sum);
            }
        }
        return ret;
    }