Return the final pair or last element of a non-empty list. Fails if the list is improper.
last-pair(1, 2, 3, 4, 5) ⇒ #(5)last-pair(1, 2, 3, #(4, 5)) ⇒ #(#(4, 5))last-pair(1, 2, 3, 4 . 5) ⇒ fails