}
  }

  /** Listeners form a Treiber stack through the {@link #listeners} field. */
  static final class Listener {
    static final Listener TOMBSTONE = new Listener();
    // null only for TOMBSTONE
    final @Nullable Runnable task;
    // null only for TOMBSTONE
    final @Nullable Executor executor;

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