public function _set_gstate($gstate_options)
    {
        if (($gstate = array_search($gstate_options, $this->_gstates)) === false) {
            $gstate = $this->_pdf->create_gstate($gstate_options);
            $this->_gstates[$gstate] = $gstate_options;
        }

        return $this->_pdf->set_gstate($gstate);
    }