public function getParameter($parameterName) {
    $parameterName = ltrim($parameterName, '$');
    $param_tags = $this->getDocBlock()->getTagsByName('param');
    /** @var \phpDocumentor\Reflection\DocBlock\Tag\ParamTag $param_tag */
    foreach ($param_tags as $param_tag) {
      if (ltrim($param_tag->getVariableName(), '$') === $parameterName) {
        return $param_tag;
      }
    }
    return NULL;
  }