default int level() {
		Optional<T> ancestor = Optional.of(self());
		int levels = 0;
		while ((ancestor = ancestor.flatMap(Tree::parent)).isPresent()) {
			++levels;
		}

		return levels;
	}
Returns the number of levels above this node. The <i>level</i> of a tree
is the distance from the root to {@code this} node. If {@code this} node
is the root, returns 0.

@return the number of levels above this node
jenetics.ext/src/main/java/io/jenetics/ext/util/Tree.java