The example they considered was removing the first element of the list, while that example is removing the last element. In terms of computational cost, this m

The example they considered was removing the first element of the list, while that example is removing the last element. In terms of computational cost, this makes sense: removing the last element of a list in lisp takes linear time with the size of the list, while removing the first element takes constant time. (If you're returning a copy this is moot, but if you're modifying in-place this could be a problem.)