;(define (make-mobile left right)
  ;(list left right))

(define (make-mobile left right)
  (cons left right))

;(define (make-branch length structure)
  ;(list length structure))

(define (make-branch length structure)
  (cons length structure))

(define (left-branch mobile)
  (car mobile))

;(define (right-branch mobile)
  ;(cadr mobile))

(define (right-branch mobile)
  (cdr mobile))

(define (branch-length branch)
  (car branch))

;(define (branch-structure branch)
  ;(cadr branch))

(define (branch-structure branch)
  (cdr branch))


(define (mobile? m) (pair? m))

(define (branch-weight branch)
  (let ((struct (branch-structure branch)))
       (if (mobile? struct)
	   (total-weight struct)
	   struct)))

(define (total-weight mobile)
  (+ (branch-weight (left-branch mobile))
     (branch-weight (right-branch mobile))))


(define (torque branch)
  (* (branch-length branch)
     (branch-weight branch)))

(define (balanced? struct)
  (cond ((null? struct) #t)
	((not (mobile? struct)) #t)
	(else 
	  (let* ((l (left-branch struct))
		 (r (right-branch struct))
		 (l-struct (branch-structure l))
		 (r-struct (branch-structure r)))
	       (and (balanced? l-struct)
		     (balanced? r-struct)
		     (= (torque l) (torque r)))))))