* real problem: The Entry objects are ImmutableMap entries, whose fields are all final and thus
     * safe to read even when the Entry object is unsafely published. But with a mutable graph, the
     * Entry object is likely to have a non-final value field, which is not safe to read when
     * unsafely published. (The Entry object might even be newly created by each iterator.next()

      new AtomicReference<>(immediateVoidFuture());

  @LazyInit private ThreadConfinedTaskQueue latestTaskQueue = new ThreadConfinedTaskQueue();

  /**
   * This object is unsafely published, but avoids problematic races by relying exclusively on the
   * identity equality of its Thread field so that the task field is only accessed by a single
   * thread.
   */