public function add_sub_module($module, $title) {
        $this->resource_count++;
        $resource_str = str_pad($this->resource_count.'',6,'0',STR_PAD_LEFT);
        $identifier = 'T_'.$resource_str;

        $sub_module = $this->add_child_ns(CC::CC_1_1_CP, $module, 'item', null, array('identifier' => $identifier));
        $new_title = $this->add_child_ns(CC::CC_1_1_CP, $sub_module, 'title',$title);
        return $sub_module;
    }