public function request($subject, $payload, $callback, $wait = 1)
{
$inbox = uniqid('_INBOX.');
$this->subscribe($inbox, $callback);
$msg = 'PUB ' . $subject . ' ' . $inbox . ' ' . strlen($payload);
$this->send($msg . "\r\n" . $payload);
$this->pubs += 1;
$this->wait($wait);
}