* guaranteed to execute. There is no guarantee, however, that listeners will be executed in the
 * order that they are added.
 *
 * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
 * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception

   * calls to {@link Executor#execute `execute()`} that submitted those tasks had.
   *
   * <p>The executor uses {@code delegate} in order to {@link Executor#execute execute} each task in
   * turn, and does not create any threads of its own.
   *
   * <p>After execution begins on a thread from the {@code delegate} {@link Executor}, tasks are

    @CheckForNull // null only for TOMBSTONE
    final Executor executor;

    // writes to next are made visible by subsequent CAS's on the listeners field
    @CheckForNull Listener next;

    Listener(Runnable task, Executor executor) {
      this.task = task;
      this.executor = executor;
    }

    Listener() {
      this.task = null;
      this.executor = null;
    }
  }

      @Override
      public Thread newThread(Runnable r) {
        return new Thread(r);
      }
    }

    public static final class DummyExecutor implements Executor, Serializable {
      @Override
      public void execute(Runnable command) {}
    }
  }

  private static final class NullByteSink extends ByteSink implements Serializable {
    private static final NullByteSink INSTANCE = new NullByteSink();

      @Override
      public Thread newThread(Runnable r) {
        return new Thread(r);
      }
    }

    public static final class DummyExecutor implements Executor, Serializable {
      @Override
      public void execute(Runnable command) {}
    }
  }

  private static final class NullByteSink extends ByteSink implements Serializable {
    private static final NullByteSink INSTANCE = new NullByteSink();