Hello. There was a post about couple of month ago about tree called close to 'velvet' that is very powerfull but not that known. And i seem unable to google that post 'cause of misspelling its name. Can anyone help me and share the link?

Given 2 mutisets A and B, |A| = |B|, |A|, |B| <= 10^5, 0 <= elements in set <= 10^18. The task is to find such X that A ^ X = B or say that such X doesnt exists. How can one do that? A ^ X means foreach element of A xor it with X

I believe this was asked already, but whats the point of it? Just curious