function addLink($url, $x0, $y0, $x1, $y1)
    {
        $this->numObj++;
        $info = array('type' => 'link', 'url' => $url, 'rect' => array($x0, $y0, $x1, $y1));
        $this->o_annotation($this->numObj, 'new', $info);
    }