iterator.go
func (it *Iterator) Prev() {
	prev := it.list.getPrev(it.nd, 0)
	it.setNode(prev, true)
}
func (it *Iterator) Prev() {
	prev := it.list.getPrev(it.nd, 0)
	it.setNode(prev, true)
}
// Prev moves to the previous position. If there are no previous nodes, then
// Valid() will be false after this call.