public function addFilter($name, $filter = null)
    {
        if (!$name instanceof Twig_SimpleFilter && !($filter instanceof Twig_SimpleFilter || $filter instanceof Twig_FilterInterface)) {
            throw new LogicException('A filter must be an instance of Twig_FilterInterface or Twig_SimpleFilter');
        }

        if ($name instanceof Twig_SimpleFilter) {
            $filter = $name;
            $name = $filter->getName();
        }

        if ($this->extensionInitialized) {
            throw new LogicException(sprintf('Unable to add filter "%s" as extensions have already been initialized.', $name));
        }

        $this->staging->addFilter($name, $filter);
    }