读到一篇新英的某段落有个看不懂地方，以下为节录
Two years ago North Korean agreed to close its main nuclear ......
and reveal its nuclear assets, but has since refused to negotiate.
问题是 but has since refused to negotiate. 请问这句的原句长什么样子
被中间那个since 搞糊涂了
请文法高手帮我解释一下了 谢谢!

http://news.bbc.co.uk/2/hi/8066615.stm 看原文就知了。