for (set >:: iterator it = a.begin(); it != a.end();/* blank */)
{
    cout << it->first << " " << it->second << endl;
    if (freq[it->first] == 1 || freq[it->second] == 1)
    {
        it = a.erase(it);
          removed = true;
    }
    else
    {
        ++it;
    }
  
}