* including those that are mapped to null values only.
   */
  public ImmutableList<C> columnKeyList() {
    return columnList;
  }

  /**
   * Returns the value corresponding to the specified row and column indices. The same value is
   * returned by {@code get(rowKeyList().get(rowIndex), columnKeyList().get(columnIndex))}, but this
   * method runs more quickly.
   *