public function get($name)
    {
        if (isset($this->_handlers[$name])) {
           return $this->_handlers[$name];
        }
        $checker = $this->_classes['checker'];
        if ($checker::has($name)) {
           return $checker::get($name);
        }
        throw new InvalidArgumentException("Unexisting `{$name}` as validation handler.");
    }