So long = farewell. So long 是近代美国人创造的词，意思就是farewell。选择哪一个完全看作者的偏好。附farewell释义：: 1. a wish of well-being at parting : GOODBYE said their farewells and headed home 2a : an act of departure : LEAVE-TAKING I will take my farewell of this place tomorrow. b : a formal occasion honoring a person about to leave or retire held a great farewell for the retiring senator