public function getManifestIndex()
    {
        $file = $this->manifestIndexFile();
        if (is_file($file)) {
            $index = Rails\Yaml\Parser::readFile($file);
            # Force array.
            if (!is_array($index)) {
                $index = [];
            }
        } else {
            $index = [];
        }
        return $index;
    }