public function getAnalysis($analyzerName, $arg = null, $startAt = 0)
    {
        $key = $analyzerName . ':' . $startAt;
        if (!array_key_exists($key, $this->analyses)) {
            $analyzer = new $analyzerName($arg);
            $analyzer->position = $startAt;
            $result = new \StdClass;
            $result->analysis = $this->runAnalyzer($analyzer);
            $result->position = $analyzer->position;
            $this->analyses[$key] = $result;
        }
        return $this->analyses[$key];
    }