public function insertVerifier($eventName, callable $verifier)
    {
        $this->attachListener($eventName, function (EventInterface $event) use ($verifier) {
            $result = $verifier($event);

            //return type hints pls
            if ($result instanceof ResultInterface) {
                $this->resultAggregator->add($result);
            } else {
                //??!!
            }
        });
    }