(define (square x) (* x x))

(define (tree-map f tree)
  (cond ((null? tree) null)
    ((not (pair? tree)) (f tree))
    (else (cons (tree-map f (car tree))
            (tree-map f (cdr tree))))))

(define (square-tree tree)
  (tree-map square tree))

(define x (list (list 1 (list 2 (list 3 4) 5) (list 6 7))))

(display x)
(newline)
(display (square-tree x))
(newline)