iterFor example

    public function __construct($iterable, array $config = [])
    {
        $this->iterable = Create::iterFor($iterable);

        if (isset($config['concurrency'])) {
            $this->concurrency = $config['concurrency'];
        }

        if (isset($config['fulfilled'])) {
            $this->onFulfilled = $config['fulfilled'];
        }

        if (isset($config['rejected'])) {
            $this->onRejected = $config['rejected'];
        }

if (!isset($config['concurrency'])) {
            $config['concurrency'] = 25;
        }

        if (isset($config['options'])) {
            $opts = $config['options'];
            unset($config['options']);
        } else {
            $opts = [];
        }

        $iterable = P\Create::iterFor($requests);
        $requests = static function D) use ($iterable, $client, $opts) {
            foreach ($iterable as $key => $rfn) {
                if ($rfn instanceof RequestInterface) {
                    yield $key => $client->sendAsync($rfn, $opts);
                } elseif (\is_callable($rfn)) {
                    yield $key => $rfn($opts);
                } else {
                    throw new \InvalidArgumentException('Each value yielded by the iterator must be a Psr7\Http\Message\RequestInterface or a callable that returns a promise that fulfills with a Psr7\Message\Http\ResponseInterface object.');
                }
            }
        };