public function moveDown(Model\ContainerInterface $container)
    {
        $sibling = $this->editor->getRepository()->findSiblingContainer($container, true);
        if (null === $sibling) {
            throw new InvalidOperationException(
                "The container can't be moved down as no sibling container has been found."
            );
        }

        $container->setPosition($container->getPosition() + 1);
        $sibling->setPosition($sibling->getPosition() - 1);

        $this->sortChildrenByPosition($container->getContent(), 'containers');

        return $sibling;
    }