const TSegment operator + (const TSegment& d) const {
    TSegment res;
    res.toadd = 0;
    res.len = len + d.len;
    res.sum = Sum() + d.Sum();
    res.ansVal = AnsVal() + d.AnsVal() + len * d.Sum();
    return res;
}

let's divide interval [2, 5] into [2, 3] and [4, 5]
1*a[2]+2*a[3]+3*a[4]+4*a[5] = 1*a[2]+2*a[3]+ 2*(a[4]+a[5]) +1*a[4]+2*a[5]