def formulas(sheet = nil)
      sheet_for(sheet).cells.select { |_, cell| cell.formula }.map do |(x, y), cell|
        [x, y, cell.formula]
      end
    end
def formulas(sheet = nil)
      sheet_for(sheet).cells.select { |_, cell| cell.formula }.map do |(x, y), cell|
        [x, y, cell.formula]
      end
    end
returns each formula in the selected sheet as an array of tuples in following format
 [[row, col, formula], [row, col, formula],...]