public static <T> PreviousHelper<T> expectPreviousElements(
     BidirectionalIterator<T> iterator, T... elements) {
   for (T element : elements) {
     assertTrue(iterator.hasPrevious());
     assertEquals(element, iterator.previous());
   }
   return new PreviousHelper<T>(iterator);
 }
 public void withNoPrevious() {
   assertFalse(iterator.hasPrevious());
 }