public function addStyle(Style $style)
    {
        if (null === $style->getId()) {
            $this->register($style->getFont(), $this->fonts);
            $this->register($style->getFill(), $this->fills);
            $this->register($style->getBorder(), $this->borders);
            $this->register($style, $this->styles);
            $style->lock();
        }
    }