这里有介绍http://cache.baidu.com/c?word=%C4%BE%E9%C8%3B%CA%F7%D2%B6%2C%CF%B4%CD%B7&url=http%3A//www%2Eu0575%2Ecom/text%5Fview%2Easp%3Fid%3D7841&b=17&a=0&user=baidu