default Iterator<T> depthFirstIterator() {
		return postorderIterator();
	}
Return an iterator that traverses the subtree rooted at {@code this} node
in depth-first order. The first node returned by the iterator is the
leftmost leaf. This is the same as a postorder traversal.
<p>
Modifying the tree by inserting, removing, or moving a node invalidates
any iterator created before the modification.

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