Hi codeforces, recently I have came across an interesting problem : https://codeforces.com/problemset/problem/617/E. I had a rough time trying to understand the solution of this problem even after i read the tutorial. Basically I can't see why the add() and del() function work because of my lack of knowledges about the XOR operation. Now after a few hours of thinking, I have officially surrendered ... That's why I'm here, asking for an explanation.

P/S : I'm just dumb, especially when it come to math and other similar stuff. Pls don't be to harsh on me ...