code: public synchronized void postAtFrontOfQueue(Runnable runnable) {
      if (isPaused() || Thread.currentThread() != associatedThread) {
        final long timeDisambiguator;
        if (runnables.isEmpty()) {
          timeDisambiguator = nextTimeDisambiguator++;
        } else {
          timeDisambiguator = runnables.peek().timeDisambiguator - 1;
        }
        runnables.add(new ScheduledRunnable(runnable, 0, timeDisambiguator));
      } else {
        runOrQueueRunnable(runnable, currentTime);
      }
    }
docs: Add a runnable to the head of the queue.
  
  @param runnable  Runnable to add.