public function toString(string $format = null) : string
    {
        $operations = $this->operations;
        $operations[] = [ 'getImage', [ $format ] ];
        
        $driver = $this->getDriver();
        return array_reduce($operations, function ($carry, $item) use ($driver) {
            return $driver->{$item[0]}(...$item[1]);
        });
    }