@Override
  public Set<Cell<R, C, V>> cellSet() {
    Set<Cell<R, C, V>> result = cellSet;
    return (result == null) ? cellSet = createCellSet() : result;
  }

  Set<Cell<R, C, V>> createCellSet() {
    return new CellSet();
  }

  abstract Iterator<Table.Cell<R, C, V>> cellIterator();

  Spliterator<Table.Cell<R, C, V>> cellSpliterator() {