;;; Procedure:
;;;   tree->code
;;; Parameters:
;;;   tree, a tree (or tree-like structure)
;;; Purpose:
;;;   Build a list that resembles the code to construct the tree.
;;; Produces:
;;;   code, a list
;;; Preconditions:
;;;   [No additional]
;;; Postconditions:
;;;   If you evaluate code, you get tree
;;; Practica:
;;;   > (tree->code (cons 'a 'b))
;;;   '(cons a b)
;;;   > (tree->code '(a . b))
;;;   '(cons a b)
;;;   > (tree->code '((a . b) c . d))
;;;   '(cons (cons a b) (cons c d))
;;;   > (tree->code (list 'a 'b 'c 'd))
;;;   '(cons a (cons b (cons c (cons d null))))
(define tree->code
  (lambda (tree)
    (cond
      [(null? tree)
       'null]
      [(pair? tree)
       (list 'cons (tree->code (car tree)) (tree->code (cdr tree)))]
      [else
       tree])))