Exercise 4.68.  Define rules to implement the reverse operation of 
exercise 2.18, which returns a list containing the same elements as a 
given list in reverse order. (Hint: Use append-to-form.) Can your rules 
answer both (reverse (1 2 3) ?x) and (reverse ?x (1 2 3)) ?

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(rule (reverse () ()))

(rule (reverse (?a . ?b) ?c)
  (and (reverse ?b ?b-rev)
       (append-to-form ?b-rev (?a) ?c)))