}
  }

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

  private final class ColumnKeyIterator extends AbstractIterator<C> {
    // Use the same map type to support TreeMaps with comparators that aren't
    // consistent with equals().
    final Map<C, V> seen = factory.get();