default Stream<T> preorderStream() {
		return StreamSupport
			.stream(spliteratorUnknownSize(preorderIterator(), 0), false);
	}
Return a stream that traverses the subtree rooted at {@code this} node
in pre-order. The first node returned by the stream is {@code this} node.
<p>
Modifying the tree by inserting, removing, or moving a node invalidates
any iterator created before the modification.

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