/**
   * The runnable, executor pairs to execute. This acts as a stack threaded through the {@link
   * RunnableExecutorPair#next} field.
   */
  @GuardedBy("this")
  @CheckForNull
  private RunnableExecutorPair runnables;

  @GuardedBy("this")
  private boolean executed;

  /** Creates a new, empty {@link ExecutionList}. */
  public ExecutionList() {}

  /**

     *   - Active: shutdown == false
     *   - Shutdown: runningTasks > 0 and shutdown == true
     *   - Terminated: runningTasks == 0 and shutdown == true
     */
    @GuardedBy("lock")
    private int runningTasks = 0;

    @GuardedBy("lock")
    private boolean shutdown = false;

    @Override
    public void execute(Runnable command) {
      startTask();
      try {
        command.run();