*
   * <p>This method can be used to traverse over a binary tree. Given methods {@code
   * leftChild(node)} and {@code rightChild(node)}, this method can be called as
   *
   * {@snippet :
   * Traverser.forTree(node -> ImmutableList.of(leftChild(node), rightChild(node)));
   * }
   *
   * @param tree {@link SuccessorsFunction} representing a directed acyclic graph that has at most