public function registerRenderable(Renderable\RenderableInterface $renderable)
    {
        if ($renderable instanceof FormElementInterface) {
            if (isset($this->elementsByIdentifier[$renderable->getIdentifier()])) {
                throw new Exception\DuplicateFormElementException(sprintf('A form element with identifier "%s" is already part of the form.', $renderable->getIdentifier()), 1325663761);
            }
            $this->elementsByIdentifier[$renderable->getIdentifier()] = $renderable;
        }
    }