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)) ?

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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)

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; rule for singleton lists:

(rule (last-pair (?x) (?x)))

; lists length ≥ 2

(rule (last-pair (?x . ?y) (last-pair ?y)))

Untested, but they look like they might work.  (Making a few assumptions 
about how these things are actually evaluated.)