protected function getPrefix()
  {
    $prefix = "";

    if(count($this->currSib))
    {
      $last = array_pop($this->currSib);

      foreach($this->currSib as $sibling)
      {
        $prefix .=  ($sibling) ? $this->getImage("tree_line.gif") : $this->getImage("tree_blank.png");
      }

      $prefix .= ($last) ? $this->getImage("tree_end.gif") : $this->getImage("tree_mid.gif");
    }

    return $prefix;
  }