private static function findByRel(array $a, $rel)
    {
        $relName = mb_strtolower($rel, 'UTF-8');
        foreach ($a as $name => $value) {
            if (mb_strtolower($name, 'UTF-8') === $relName) {
                return $value;
            }
        }
        
        return null;
    }