function check_options(el)
{
  var size = el.options.length;
  var value;
  var select;
  var selected;
  var all = false;
  
  for (i = 0; i < size; i++)
  {
    value    = el.options[i].value;
    selected = el.options[i].selected;  
    
    if (value == 0 && selected)
    { // vse
      all = true;
    }
    else if (!all && value == 'o')
    {         
      if (selected == true)
      {
        select = true;
        el.options[i].selected = false;
      }
      else
      {
        select = false;
      }
    }
    else if (all)
    {            
      el.options[i].selected = false;
    }
    else
    {
      el.options[i].selected = el.options[i].selected || select;
    }
  }
} // chech_options()