* those that are mapped to null values only.
   */
  public ImmutableList<R> rowKeyList() {
    return rowList;
  }

  /**
   * Returns, as an immutable list, the column keys provided when the table was constructed,
   * including those that are mapped to null values only.
   */
  public ImmutableList<C> columnKeyList() {
    return columnList;

   * those that are mapped to null values only.
   */
  public ImmutableList<R> rowKeyList() {
    return rowList;
  }

  /**
   * Returns, as an immutable list, the column keys provided when the table was constructed,
   * including those that are mapped to null values only.
   */
  public ImmutableList<C> columnKeyList() {
    return columnList;