public function __isset($helper)
    {
        if (isset($this->_loaded[$helper])) {
            return true;
        }

        try {
            $this->load($helper);
        } catch (Exception\MissingHelperException $exception) {
            if ($this->_View->getPlugin()) {
                $this->load($this->_View->getPlugin() . '.' . $helper);

                return true;
            }
        }

        if (!empty($exception)) {
            throw $exception;
        }

        return true;
    }