EX 2.63

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

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

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

(define (tree->list-1 tree)
  (if (null? tree)
      '()
      (append (tree->list-1 (left-branch tree))
              (cons (entry tree)
                    (tree->list-1 (right-branch tree))))))

(define (tree->list-2 tree)
  (define (copy-to-list tree result-list)
    (if (null? tree)
        result-list
        (copy-to-list (left-branch tree)
                      (cons (entry tree)
                            (copy-to-list (right-branch tree)
                                          result-list)))))
  (copy-to-list tree '()))

;(null? nil) in MIT scheme is true but in guile is false
;(null? '()) both is true.

(define tree1 (make-tree 7
                        (make-tree 3
                                   (make-tree 1 '() '())
                                   (make-tree 5 '() '()))
                        (make-tree 9 '()
                                   (make-tree 11 '() '())
                                   )
                        )
  )

(define tree2 (make-tree 3
                        (make-tree 1 '() '())