She is keen for Britain tojoin the Comprehensive and Progressive Agreement for Trans-Pacific Partnership,a trade deal of modest economic value but inwhich she sees a bulwark against Russia.
请教老师这句话的but是并列连词吧，a trade deal ofmodest economic value 是同位语，in which的先行词是Agreement ，but前是否应该加逗号？
She is keen for Britain to join the Comprehensive and Progressive Agreement for Trans-Pacific Partnership, a trade deal of modest economic value but in which she sees a bulwark against Russia.
非限制性同位语和可以视为非限制性定语从句的简略。假如没后面的but in which, 这个同位语前加或不加which is都是可以的，直接用名词短语作同位语更为简练。但因为后面有定语从句，这个which is的简略就欠妥了，使得并列连词but把一个名词性结构和一个形容词性结构并列，违反了平行原则。这是作者写作或说话时欠考虑致使的写作败笔。大家理解的时候在同位语前加上which is, 如此but并列二个定语从句，这二个定语从句之间具备转折关系，第一个定语从句说明该协定经济价值不高，第二个定语从句说明它是对抗俄罗斯的堡垒，有明显的转折关系。所以，第二个定语从句的which的先行词应该是the Comprehensive and Progressive Agreement for Trans-Pacific Partnership。
