public static function getWritableDirs(Event $event)
	{
		$configuration = $event->getComposer()->getPackage()->getExtra();
		if (!isset($configuration['writable-dirs'])) {
			throw new \Exception('The writable-dirs must be specified in composer arbitrary extra data.');
		}
		if (!is_array($configuration['writable-dirs'])) {
			throw new \Exception('The writable-dirs must be an array.');
		}
		return $configuration['writable-dirs'];
	}