Exercise 2.17: Define a procedure last-pair that returns the list that contains only the last element
of a given (nonempty) list:
(last-pair (list 23 72 149 34))
(34)
(define (last-pair elems)
(if (null? (cdr elems))
elems
(last-pair (cdr elems))))
> (last-pair (list 1 2 3 4 5))
(5)