protected function getPartialSource($partialName)
    {
        $partialPathAndFilename = $this->getPartialPathAndFilename($partialName);
        $partialSource = Files::getFileContents($partialPathAndFilename, FILE_TEXT);
        if ($partialSource === false) {
            throw new Exception\InvalidTemplateResourceException('"' . $partialPathAndFilename . '" is not a valid template resource URI.', 1257246929);
        }
        return $partialSource;
    }