function tcb_checkPriorityAdd(task, packet) {
  if (this.queue == null) {
    this.queue = packet;
    tcb_markAsRunnable.call(this);
    if (this.priority > task.priority) return this;
  } else {
    this.queue = packet_addTo.call(packet, this.queue);
  }
  return task;
}
Adds a packet to the worklist of this block's task, marks this as runnable if
necessary, and returns the next runnable object to run (the one
with the highest priority).