public function focus($frameId=null)
    {
        if ($frameId instanceof WebDriver_Element) {
            $frameId = $frameId->getReference();
        }
        $params = [
            'id' => $frameId
        ];
        $command = $this->driver->factoryCommand(
            'frame',
            WebDriver_Command::METHOD_POST,
            $params
        );
        return $this->driver->curl($command)['value'];
    }