if (log.isTraceEnabled()) { log.trace(this + " cancelling delivery"); }
if (trace) { log.trace(this + " cancelling delivery"); }
   public synchronized void cancel() throws Throwable   {      if (log.isTraceEnabled()) { log.trace(this + " cancelling delivery"); }            // deals with the race condition when cancellation arrives before the delivery      // is returned back to the sending delivery observer            observer.cancel(this);      cancelled = true;   }