* need. Both fields are guarded by `this`.
   */
  private var executeCallCount = 0
  private var runCallCount = 0

  /** Queues with tasks that are currently executing their [TaskQueue.activeTask]. */
  private val busyQueues = mutableListOf<TaskQueue>()

  /** Queues not in [busyQueues] that have non-empty [TaskQueue.futureTasks]. */
  private val readyQueues = mutableListOf<TaskQueue>()