iterator.go
func (it *Iterator) Next() {
	next := it.list.getNext(it.nd, 0)
	it.setNode(next, false)
}
func (it *Iterator) Next() {
	next := it.list.getNext(it.nd, 0)
	it.setNode(next, false)
}
// Next advances to the next position. If there are no following nodes, then
// Valid() will be false after this call.