private function addNextRequest()
    {
        add_next:

        if ($this->isRealized || !$this->iter || !$this->iter->valid()) {
            return false;
        }

        $request = $this->iter->current();
        $this->iter->next();

        if (!($request instanceof RequestInterface)) {
            throw new \InvalidArgumentException(sprintf(
                'All requests in the provided iterator must implement '
                . 'RequestInterface. Found %s',
                Core::describeType($request)
            ));
        }

        // Be sure to use "lazy" futures, meaning they do not send right away.
        $request->getConfig()->set('future', 'lazy');
        $hash = spl_object_hash($request);
        $this->attachListeners($request, $this->eventListeners);
        $request->getEmitter()->on('before', [$this, '_trackRetries'], RequestEvents::EARLY);
        $response = $this->client->send($request);
        $this->waitQueue[$hash] = $response;
        $promise = $response->promise();

        // Don't recursively call itself for completed or rejected responses.
        if ($promise instanceof FulfilledPromise
            || $promise instanceof RejectedPromise
        ) {
            try {
                $this->finishResponse($request, $response->wait(), $hash);
            } catch (\Exception $e) {
                $this->finishResponse($request, $e, $hash);
            }
            goto add_next;
        }

        // Use this function for both resolution and rejection.
        $thenFn = function ($value) use ($request, $hash) {
            $this->finishResponse($request, $value, $hash);
            if (!$request->getConfig()->get('_pool_retries')) {
                $this->addNextRequests();
            }
        };

        $promise->then($thenFn, $thenFn);

        return true;
    }