@Override
	default Iterator<T> iterator() {
		return breadthFirstIterator();
	}
Return an iterator that traverses the subtree rooted at {@code this}.
The first node returned by the iterator is {@code this} node.
<p>
Modifying the tree by inserting, removing, or moving a node invalidates
any iterator created before the modification.

@see #breadthFirstIterator
@return an iterator for traversing the tree in breadth-first order
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java