docs: public void cancelRequest(long ticketNumber) {
      if (this.session != null
          && this.outstandingRequests.containsKey(Long.valueOf(ticketNumber))) {
        CancelRequestMessage message = new CancelRequestMessage();
        message.setTicketNumber(ticketNumber);
        this.session.write(message);
      } else {
        log.warn("Tried to cancel unknown request for ticket number {}.",
            Long.valueOf(ticketNumber));
      }
    }