#define it std::set<node>::iterator

void update(int l, int r) {
  it R = split(r);
  it L = split(i);
  for (it i = L; i != R;) {
      it j = std::next(i);
      ll tr = i->r;
      while (j != R && i->b == j->b) {
          tr = j->r;
          j = s.erase(j);
      }
      i->r = tr;
      /* update... */
      i = j;
  }
}

感谢