(define (mk-ackumulator)
    (let ((l '()))
        (case-lambda
            (()  (reverse! l))
            ((x) (set! l (cons (list x) l)))))

(define (tree-traversal x ack)
    (if (pair? x)
        (begin
            (tree-traversal (car x) ack)
            (tree-traversal (cdr x) ack))
        (if (null? x)
            (values)
            (ack x))))

(define (optimized-flatten x) (tree-traversal x (mk-ackumulator)))