def worksheet idx
      case idx
      when Integer
        @worksheets[idx]
      when String
        @worksheets.find do |sheet| sheet.name == idx end
      end
    end
def worksheet idx
      case idx
      when Integer
        @worksheets[idx]
      when String
        @worksheets.find do |sheet| sheet.name == idx end
      end
    end
The Worksheet at _idx_, or - if _idx_ is a String -
 the Worksheet with name == _idx_