mobile.scm

(define (make-mobile left right)
  (list left right))
(define (make-branch length structure)
  (list length structure))
(define (left-branch x)
  (car x))
(define (right-branch x)
  (cadr x))
(define (branch-length x)
  (car x))
(define (branch-structure x)
  (cadr x))
(define (total-weight x)
  (cond ((null? x) 0)
	((not (pair? x)) x)
	(else (+ (total-weight (left-branch x))
		 (total-weight (right-branch x))))))
(define (balanced? x)
  (cond ((null? x) #t)
	((= (* (branch-length (left-branch x)) (total-weight (left-branch x)))
	    (* (branch-length (right-branch x)) (total-weight (right-branch x))))
	 (and (balanced? (left-branch x))
	      (balanced? (right-branch x))))
	(else #f)))
;; d
cadr -> cdr