public UnmodifiableIterator<T> iterator() {
        return preOrderIterator(root);
      }
    };
  }

  UnmodifiableIterator<T> preOrderIterator(T root) {
    return new PreOrderIterator(root);
  }

  private final class PreOrderIterator extends UnmodifiableIterator<T> {
    private final Deque<Iterator<T>> stack;

    PreOrderIterator(T root) {
      this.stack = new ArrayDeque<>();

FIELD com.google.common.collect.SingletonImmutableBiMap.inverse
FIELD com.google.common.collect.TreeTraverser.PostOrderNode.childIterator
FIELD com.google.common.collect.TreeTraverser.PreOrderIterator.stack
FIELD com.google.common.util.concurrent.AbstractFuture.Listener.executor com.google.common.util.concurrent.MoreExecutors.rejectionPropagatingExecutor.$
FIELD com.google.common.util.concurrent.AbstractService.listeners