protected function convertExtraKey(array $asset, $assetKey, array &$composer, $composerKey, $extraKey = 'extra')
    {
        $extra = isset($composer[$extraKey]) ? $composer[$extraKey] : array();

        $this->convertKey($asset, $assetKey, $extra, $composerKey);

        if (\count($extra) > 0) {
            $composer[$extraKey] = $extra;
        }
    }