public function export($axml)
    {
        $bset = $axml->addChild('blocktypeset');
        $bset->addAttribute('handle', $this->getBlockTypeSetHandle());
        $bset->addAttribute('name', $this->getBlockTypeSetName());
        $bset->addAttribute('package', $this->getPackageHandle());
        $types = $this->getBlockTypes();
        foreach ($types as $bt) {
            $typenode = $bset->addChild('blocktype');
            $typenode->addAttribute('handle', $bt->getBlockTypeHandle());
        }

        return $bset;
    }