Method add_or

code/other/two_sat.cpp:10–11

  bool put(int x, int v) {
    return (V[n+x].val &= v) != (V[n-x].val &= 1-v); }
  void add_or(int x, int y) {
    V[n-x].adj.push_back(n+y), V[n-y].adj.push_back(n+x); }
  int dfs(int u) {
    int br = 2, res;
    S.push_back(u), V[u].num = V[u].lo = at++;