var ajax_request = function(/*url*/)
  {
    var url = '?subjects_list';
    var request = new XMLHttpRequest();
    request.open('GET', url);
    request.onreadystatechange = function()
    {
      try
      {
        if(request.readyState == 4)
          if(request.status == 200)
            // Answer
            return request.responseText;
          else
          {
            console.log('invalid response: ' + request.statusText);
          }
      }
      catch(e)
      {
        console.log('request error: ' + e.description);
      }
    }
    request.send(null);
  }