}
    };
  }

  UnmodifiableIterator<T> postOrderIterator(T root) {
    return new PostOrderIterator(root);
  }

  private static final class PostOrderNode<T> {
    final T root;
    final Iterator<T> childIterator;

    PostOrderNode(T root, Iterator<T> childIterator) {
      this.root = checkNotNull(root);
      this.childIterator = checkNotNull(childIterator);
    }
  }

FIELD com.google.common.collect.Maps.FilteredMapValues.unfiltered
FIELD com.google.common.collect.Sets.SubSet.inputSet
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.$