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

; 1 ]=> (square-tree
;         (list 1
;                (list 2 (list 3 4) 5)
;                (list 6 7)))
; 
; ;Value: (1 (4 (9 16) 25) (36 49))

; Alternative implementation
(define (square-tree tree)
  (if (number? tree)
      (square tree)
      (map square-tree tree)))