def single_cell ref
      if ref.sheet_name.nil?
        raise 'cell references at the workbook level must include a sheet reference (eg. Sheet1!A1)'
      else
        worksheets[ref.sheet_name][ref.row][ref.col]
      end
    end
def single_cell ref
      if ref.sheet_name.nil?
        raise 'cell references at the workbook level must include a sheet reference (eg. Sheet1!A1)'
      else
        worksheets[ref.sheet_name][ref.row][ref.col]
      end
    end
ref is a POI::CELL_REF instance