public function exists($func)
    {
        if (!is_callable($func, false, $callable_name))
            throw new \InvalidArgumentException(get_class($this).'::exists - Un-callable "$func" value seen!');

        foreach($this->_storage as $key => $value)
        {
            if (call_user_func($func, $key, $value))
                return true;
        }

        return false;
    }