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

; (define (tree-map f tree)
;   (if (list? tree)
;        (map (lambda (child) (tree-map f child))
;             tree)
;        (f tree)))

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