checkForConcurrentModification();
      if (next == null) {
        throw new NoSuchElementException();
      }
      current = next;
      seenKeys.add(current.key);
      do { // skip ahead to next unseen key
        next = next.next;
      } while ((next != null) && !seenKeys.add(next.key));
      return current.key;
    }

    @Override
    public void remove() {
      checkForConcurrentModification();