in this problem : / *https://codeforces.net/contest/1614/problem/C */

can any body explain why ((in the editorial he multiplied x at 2 to power n-1 to get the sum of (The Xoring of each subsequence). I understand until this part ("Thus, the i -th bit is included in exactly 2|A|⋅2|B|−1=2n−1 subsequences, which was required to be proved.")