public function down()
    {
        $def = $this->table('#__bpmn_process_definition');
        $def->removeColumn('resource_id');
        $def->update();
        
        $subscription = $this->table('#__bpmn_event_subscription');
        $subscription->removeColumn('job_id');
        $subscription->removeColumn('boundary');
        $subscription->update();
        
        $this->dropTable('#__bpmn_job');
    }