public function all($username, $repository, $issue = null)
    {
        if ($issue === null) {
            $path = '/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/labels';
        } else {
            $path = '/repos/'.rawurlencode($username).'/'.rawurlencode($repository).'/issues/'.rawurlencode($issue).'/labels';
        }

        return $this->get($path);
    }