Exercise 4.62: Define rules to implement the
last-pair operation of Exercise 2.17, which returns a list
containing the last element of a nonempty list.  Check your rules on queries
such as (last-pair (3) ?x), (last-pair (1 2 3) ?x) and
(last-pair (2 ?x) (3)).  Do your rules work correctly on queries such as
(last-pair ?x (3))?