Gregwar\RST\Environment::canonicalize PHP Method

    protected function canonicalize($url)
    {
        $parts = explode('/', $url);
        $stack = array();
        foreach ($parts as $part) {
            if ($part == '..') {
                array_pop($stack);
            } else {
                $stack[] = $part;
            }
        }
        return implode('/', $stack);
    }