public function getAvailableHtmlHeadEntrySet()
    {
        $set = parent::getAvailableHtmlHeadEntrySet();

        if ($this->using_null_replication) {
            $set->addEntrySet(
                $this->getPrototypeWidget()->getAvailableHtmlHeadEntrySet()
            );
        } else {
            $widgets = $this->getWidgets();
            foreach ($widgets as $widget) {
                $set->addEntrySet($widget->getAvailableHtmlHeadEntrySet());
            }
        }

        return $set;
    }