}
  }

  fun cancelAll() {
    lock.assertHeld()
    for (i in busyQueues.size - 1 downTo 0) {
      busyQueues[i].cancelAllAndDecide()
    }
    for (i in readyQueues.size - 1 downTo 0) {
      val queue = readyQueues[i]
      queue.cancelAllAndDecide()
      if (queue.futureTasks.isEmpty()) {
        readyQueues.removeAt(i)
      }
    }
  }

  interface Backend {