(define-struct node (left right))
(define-struct pair (node updated))

(define (read-node a)
 (cond
  [(equal? (node-left (pair-node a)) (node-right (pair-node a))) (pair-node a)]
  [(and (= (pair-updated a) 0) (node? (node-left (pair-node a))) (eq? (node-right (pair-node a)) '())) (make-node (read-node (make-pair (node-left (pair-node a)) (+ (pair-updated a) 1))) (make-node '() '()))]
  [(and (= (pair-updated a) 0) (node? (node-right (pair-node a))) (eq? (node-left (pair-node a)) '())) (make-node (make-node '() '()) (read-node (make-pair (node-right (pair-node a)) (+ (pair-updated a) 1))))]
  [(and (= (pair-updated a) 0) (node? (node-left (node-left (pair-node a)))) (eq? (node-left (node-right (pair-node a))) '())) (make-node (read-node (make-pair (node-left (pair-node a)) (+ (pair-updated a) 1))) (make-node (make-node '() '()) '()))]
  [(and (= (pair-updated a) 0) (node? (node-left (node-right (pair-node a)))) (eq? (node-left (node-left (pair-node a))) '())) (make-node (make-node (make-node '() '()) '()) (read-node (make-pair (node-right (pair-node a)) (+ (pair-updated a) 1))))]
  [(node? (pair-node a)) (make-node (read-node (make-pair (node-left (pair-node a)) (pair-updated a))) (read-node (make-pair (node-right (pair-node a)) (pair-updated a))))]
  )
  )

(define a (make-node (make-node '() '()) '()))
(define b (make-node (make-node '() '()) (make-node '() '())))
(define c (make-node (make-node '() '()) (make-node (make-node '() '()) (make-node '() '()))))

(define a-pair (make-pair a 0))
(define b-pair (make-pair b 0))
(define c-pair (make-pair c 0))


(read-node a-pair)