protected static function mustBeArray($arg, $arg_name = null)
    {
        $msg = sprintf(
            '%s must be a primitive integer indexed array PHP type or '
            .'object \Malenki\Bah\A',
            is_null($arg_name) ? 'Argument' : $arg_name
        );

        if(!is_array($arg) && !($arg instanceof A)){
            throw new \InvalidArgumentException($msg);
        }

        if(is_array($arg)){
            foreach(array_keys($arg) as $ak){
                if(!is_integer($ak)){
                    throw new \InvalidArgumentException($msg);
                }
            }
        }
    }