}

  private static final TreeTraverser<BinaryNode> VIEWER =
      new TreeTraverser<BinaryNode>() {
        @Override
        public Iterable<BinaryNode> children(BinaryNode root) {
          return Optional.presentInstances(ImmutableList.of(root.left, root.right));
        }
      };

  enum Traversal {
    PRE_ORDER {
      @Override

Alice was not going to do THAT in a hurry.  `No, I'll look
first,' she said, `and see whether it's marked "poison" or not';
for she had read several nice little histories about children who
had got burnt, and eaten up by wild beasts and other unpleasant
things, all because they WOULD not remember the simple rules
their friends had taught them:  such as, that a red-hot poker