Exercise 2.66.  Implement the lookup procedure for the case where the 
set of records is structured as a binary tree, ordered by the numerical 
values of the keys.

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(define (lookup given-key set-of-records)
  (if (null? set-of-records)
      false
      (let ((k (key (entry set-of-records))))
        (cond ((= k given-key) (entry set-of-records))
              ((> k given-key) (lookup given-key (left-branch set-of-records)))
              (else            (lookup given-key (right-branch set-of-records)))))))