void del(int x, int c) {
assert(col[c].count(x));
int a = prv(x, c);
int b = nxt(x, c);
bit.del(x, b);
if (a != -1) {
bit.del(a, x);
bit.add(a, b);
}
col[c].erase(x);
}
void add(int x, int c) {
assert(!col[c].count(x));
int a = prv(x, c);
int b = nxt(x, c);
if (a != -1) {
bit.del(a, b);
bit.add(a, x);
}
bit.add(x, b);
col[c].insert(x);
}