(define (avl-tree-traverse origin stack key cont)
  (let loop ((parent '() ) (child origin) )
    (if (null? child)
        (cont stack parent child)
        (let ((child-key (avl-node-key child) )
              (loop-helper
               (lambda (subtree)
                 (if (not (null? parent) )
                     (avl-stack-push! stack (cons parent subtree) ) )
                 (loop child (avl-node-subtree child subtree) ) ) ) )
          (cond ((< key child-key) (loop-helper *l-subtree*) )
                ((> key child-key) (loop-helper *r-subtree*) )
                (else (cont stack parent child) ) ) ) ) ) )