def formula(row, col, sheet = nil)
      sheet ||= default_sheet
      read_cells(sheet)
      row, col = normalize(row, col)
      @formula[sheet][[row, col]]
    end
def formula(row, col, sheet = nil)
      sheet ||= default_sheet
      read_cells(sheet)
      row, col = normalize(row, col)
      @formula[sheet][[row, col]]
    end
Returns the formula at (row,col).
 Returns nil if there is no formula.
 The method #formula? checks if there is a formula.