default Stream<T> postorderStream() {
		return StreamSupport
			.stream(spliteratorUnknownSize(postorderIterator(), 0), false);
	}
Return a stream that traverses the subtree rooted at {@code this} node in
post-order. The first node returned by the iterator is the leftmost leaf.
This is the same as a depth-first traversal.

@see #depthFirstIterator
@see #preorderIterator
@return a stream for traversing the tree in post-order
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java