 to group headers.
     */
    String groupHeaderCell();
  }

  /**
   * Renders custom table headers. The top header row includes the groups "Name"
   * and "Information", each of which spans multiple columns. The second row of
   * the headers includes the contacts' first and last names grouped under the
   * "Name" category. The second row also includes the age, category, and
   * address of the contacts grouped under the "Information" category.
   */
  private class CustomHeaderBuilder extends AbstractH