code: public Stream<FlatTreeNode<T>> stream() {
  		return IntStream.range(0, _nodes.size()).mapToObj(this::nodeAt);
  	}
docs: Return a stream of all nodes of the whole underlying tree. This method
  call is equivalent to
  <pre>{@code
  final Stream<FlatTreeNode<T>> nodes = getRoot().breadthFirstStream();
  }</pre>
  
  @return a stream of all nodes of the whole underlying tree