Exercise 2.61.  Give an implementation of adjoin-set using the ordered 
representation. By analogy with element-of-set? show how to take 
advantage of the ordering to produce a procedure that requires on the 
average about half as many steps as with the unordered representation.

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(define (adjoin-set x set)
  (cond ((null? set) (list x))
        ((= x (car set)) set)
        ((< x (car set)) (cons x set))
        (else (cons (car set) (adjoin-set x (cdr set))))))