text: std::vector<int> v1 = {1, 2, 3, 4, 5};
  std::vector<int> v2 = {      3, 4, 5, 6, 7};
  std::vector<int> dest1;
  
  std::set_union(v1.begin(), v1.end(),
                 v2.begin(), v2.end(),
                 std::back_inserter(dest1));
  