用Ruby spreadsheet/excel 生成Excel文档(附问题)

require "rubygems"
require "spreadsheet/excel"
include Spreadsheet

workbook = Excel.new("test.xls")

format = Format.new
format.color = "green"
format.bold = true

worksheet = workbook.add_worksheet
worksheet.write(0, 0, "Hello", format)
worksheet.write(1, 1, ["Matz","Larry","Guido"])

workbook.close

这是doc里的例子，但是format并不能应用到"Hello"这个格子上，需要用format = workbook.add_format(:color => "green", :bold => true)才能真正生效，有知道原因的朋友可以回帖知会一下，谢谢。
PS:　这个lib只能生成，读取用parseexcel这个lib。