Having looked again, the authors do explain why they believe that continuations (as opposed to simply higher-order functions) are necessary. See my other comme

Having looked again, the authors do explain why they believe that continuations (as opposed to simply higher-order functions) are necessary. See my other comment at https://news.ycombinator.com/item?id=22347428 https://news.ycombinator.com/item?id=22347428.