#lang sicp

(define (entry tree) (car tree))

(define (left-branch tree) (cadr tree))

(define (right-branch tree) (caddr tree))

(define (make-tree entry left right)
  (list entry left right))

(define (element-of-set? x set)
  (cond ((null? set) false)
        ((= x (entry set)) true)
        ((< x (entry set))
         (element-of-set? x (left-branch set)))
        ((> x (entry set))
         (element-of-set? x (right-branch set)))))

(define key car)

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

(define records
  (make-tree '(3 three)
             (make-tree '(1 one) '() '())
             (make-tree '(5 five)
                        (make-tree '(4 four) '() '())
                        (make-tree '(7 seven) '() '()))))