如果你符合中美treaty规定（以博后身份从中国入境美国，而非在美国由其他身份转成
博后，且是在学校或公益科研机构做研究），那么即使有绿卡也能享受treaty http://www.irs.gov/publications/p519/ch09.html#en_US_2012_publi
Some exceptions to the saving clause apply to all resident aliens (for
example, under the U.S.-People's Republic of China treaty); others apply
only to resident aliens who are not lawful permanent residents of the United
States (green card holders).
