@Override
	public boolean hasNext() {
		final Iterator<T> peek = _deque.peek();
		return peek != null && peek.hasNext();
	}
Create a new preorder iterator of the given tree {@code root}.

@param root the root node of the tree
@throws NullPointerException if the given {@code root} node is
       {@code null}
jenetics.ext/src/main/java/io/jenetics/ext/util/TreeNodePreorderIterator.java