}

    @Override
    public boolean contains(@Nullable Object obj) {
      return containsColumn(obj);
    }
  }

  /** Creates an iterator that returns each column value with duplicates omitted. */
  Iterator<C> createColumnKeyIterator() {
    return new ColumnKeyIterator();
  }

  private final class ColumnKeyIterator extends AbstractIterator<C> {

    }

    @Override
    public boolean contains(@Nullable Object obj) {
      return containsColumn(obj);
    }
  }

  /** Creates an iterator that returns each column value with duplicates omitted. */
  Iterator<C> createColumnKeyIterator() {
    return new ColumnKeyIterator();
  }

  private final class ColumnKeyIterator extends AbstractIterator<C> {

   * whose elements are the result of applying {@code elementFunction} to the inputs, with counts
   * equal to the result of applying {@code countFunction} to the inputs.
   *
   * <p>If the mapped elements contain duplicates (according to {@code comparator}), the first
   * occurrence in encounter order appears in the resulting multiset, with count equal to the sum of

    assertIntact(q);
    q.remove(9);
    q.remove(11);
    q.remove(10);
    // Now we're in the critical state: [1, 15, 13, 8, 14]
    // Removing 8 while iterating caused duplicates in iteration result.
    List<Integer> result = Lists.newArrayListWithCapacity(initial.size());
    for (Iterator<Integer> iter = q.iterator(); iter.hasNext(); ) {
      Integer value = iter.next();
      result.add(value);

    private int cursor = -1;
    private int nextCursor = -1;
    private int expectedModCount = modCount;
    // The same element is not allowed in both forgetMeNot and skipMe, but duplicates are allowed in
    // either of them, up to the same multiplicity as the queue.
    private @Nullable Queue<E> forgetMeNot;
    private @Nullable List<E> skipMe;
    private @Nullable E lastFromForgetMeNot;

    private int cursor = -1;
    private int nextCursor = -1;
    private int expectedModCount = modCount;
    // The same element is not allowed in both forgetMeNot and skipMe, but duplicates are allowed in
    // either of them, up to the same multiplicity as the queue.
    private @Nullable Queue<E> forgetMeNot;
    private @Nullable List<E> skipMe;
    private @Nullable E lastFromForgetMeNot;

    assertIntact(q);
    q.remove(9);
    q.remove(11);
    q.remove(10);
    // Now we're in the critical state: [1, 15, 13, 8, 14]
    // Removing 8 while iterating caused duplicates in iteration result.
    List<Integer> result = Lists.newArrayListWithCapacity(initial.size());
    for (Iterator<Integer> iter = q.iterator(); iter.hasNext(); ) {
      Integer value = iter.next();
      result.add(value);

   * inputs, with counts equal to the result of applying {@code countFunction} to the inputs.
   * Elements are added in encounter order.
   *
   * <p>If the mapped elements contain duplicates (according to {@link Object#equals}), the element
   * will be added more than once, with the count summed over all appearances of the element.
   *

   * inputs, with counts equal to the result of applying {@code countFunction} to the inputs.
   * Elements are added in encounter order.
   *
   * <p>If the mapped elements contain duplicates (according to {@link Object#equals}), the element
   * will be added more than once, with the count summed over all appearances of the element.
   *

   * whose elements are the result of applying {@code elementFunction} to the inputs, with counts
   * equal to the result of applying {@code countFunction} to the inputs.
   *
   * <p>If the mapped elements contain duplicates (according to {@code comparator}), the first
   * occurrence in encounter order appears in the resulting multiset, with count equal to the sum of