how to find the subsequences of an array

vector<vector<int>> subsequences(vector<int> a) {
  vector<vector<int>> sol;
  // base case
  if (a.empty()){
    sol.push_back({});
    return sol;
  }
  int n = a.size();
  int last = a[n - 1];
  a.pop_back();
  vector<vector<int>> smaller = subsequences(a);
  for (auto i : smaller) {
      sol.push_back(i);
  }
  for (auto i : smaller) {
    vector<int> temp = i;
    temp.push_back(last);
    sol.push_back(temp);
  }
  return sol;
}


int main() {
  vector<int> a = {1, 2, 3, 4};
  vector<vector<int>> sol = subsequences(a);
  for (auto i : sol) {
    for (auto j : i) {
      cout << j << " ";
    }
    cout << end