public function add(PromiseCore $core): void
    {
        foreach ($this->cores as $existed) {
            if ($existed === $core) {
                return;
            }
        }

        $this->cores[] = $core;

        if (null === $this->multiHandle) {
            $this->multiHandle = curl_multi_init();
        }
        curl_multi_add_handle($this->multiHandle, $core->getHandle());
    }