public final function get_export_config($key) {
        $allowed = array_merge(
            array('hidewait', 'wait', 'format', 'hideformat'),
            $this->get_allowed_export_config()
        );
        if (!in_array($key, $allowed)) {
            $a = (object)array('property' => $key, 'class' => get_class($this));
            throw new portfolio_export_exception($this->get('exporter'), 'invalidexportproperty', 'portfolio', null, $a);
        }
        if (!array_key_exists($key, $this->exportconfig)) {
            return null;
        }
        return $this->exportconfig[$key];
    }