@CheckForNull Executor nextExecutor;
  }

  /**
   * Enqueues a task to run when the previous task (if any) completes.
   *
   * <p>Cancellation does not propagate from the output future to a callable that has begun to
   * execute, but if the output future is cancelled before {@link Callable#call()} is invoked,
   * {@link Callable#call()} will not be invoked.
   */
  public <T extends @Nullable Object> ListenableFuture<T> submit(