public function nextAsync( callable $onFulfilled = null, callable $onRejected = null, string $group = "default" ): self
	{
		$client = @$this->async_clients[$group];
		if (!$client)
			//  TODO: Guzzle Client settings?
			$this->async_clients[$group] = $client = new Client();

		$this->async_requests[$group][] = $this->next_async_request = new AsyncRequest($client);
		$this->next_async_request->onFulfilled = $onFulfilled;
		$this->next_async_request->onRejected = $onRejected;

		return $this;
	}