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();

        @Nullable Object[] localAlternatingKeysAndValues, int size) {
      Set<Object> seenKeys = new HashSet<>();
      BitSet dups = new BitSet(); // slots that are overridden by a later duplicate key
      for (int i = size - 1; i >= 0; i--) {
        Object key = requireNonNull(localAlternatingKeysAndValues[2 * i]);
        if (!seenKeys.add(key)) {
          dups.set(i);
        }
      }
      if (dups.isEmpty()) {