;;; #t
(define t7 '(6 (3 (1 () ()) (5 () ())) (10 (8 (7 () ()) (9 ()())) (12 (11 ()()) (13 ()())))))

(define complete?
  (lambda (ls)
    (cond
     [(null? ls) #t]
     [(equal? (height (cadr ls)) (height (caddr ls)))
      (and (complete? (cadr ls)) (complete? (caddr ls)))]
     [else #f])))

(define height
  (lambda (ls)
    (cond
     [(null? ls)0]
     [(number? ls) 0]
     [else (+ 1 (height (car ls)))])))