public function ack($deviceUUID, array $ackKeys, array $options = array())
  {
    // @throws InvalidArgumentException
    $this->checkArgument($deviceUUID, 'deviceUUID');

    // fast path - nothing to acknowledge
    if (!$ackKeys) return true;

    $attributes = ['ack_keys' => $ackKeys];
    list($code,) = $this->httpClient->post('/sync/ack', $attributes, array_merge($options, ['headers' => $this->buildHeaders($deviceUUID)]));
    return $code == 202;
  }