(define ob2
  (class private
 (a 10) (b 20)
 public
 (c '())
 (get-a (lambda (x) a))
 (get-b (lambda (x) b))
 (set-a! (lambda (x) (set! a x)))
 (set-b! (lambda (x) (set! b x)))
 (push-c! (lambda (x)
    (set! c (cons x c))))
 (pop-c! (lambda (x)
   (if (not (eq? c '()))
       (set! c (cdr c))
       (error "stack empty!"))))))

Error: during expansion of (class ...) - no rule matches form: (class private (a 10) (b 20) public (c (quote ())) (get-a (lambda (x) a)) (get-b (lambda (x) b)) (set-a! (lambda (x) (set! a x))) (set-b! (lambda (x) (set! b x))) (push-c! (lambda (x) (set! c (cons x c)))) (pop-c! (lambda (x) (if (not (eq? c (quote ()))) (set! c (cdr c)) (error "stack empty!")))))

        Call history:

        <eval>    (##sys#cdr l190)
        <eval>    (##sys#+ len191 -1)
        <eval>    (##sys#= len191 3)
        <eval>    (##sys#car l190)
        <eval>    (##sys#pair? temp200)
        <eval>    (##sys#cdr temp200)
        <eval>    (##sys#pair? temp200)
        <eval>    (##sys#eq? (##sys#cdr temp200) (quote ()))
        <eval>    (##sys#cdr temp200)
        <eval>    (loop195 (##sys#cdr l190) (##sys#+ len191 -1))
        <eval>    (##sys#cdr l190)
        <eval>    (##sys#+ len191 -1)
        <eval>    (##sys#= len191 3)
        <eval>    (##sys#car l190)
        <eval>    (##sys#pair? temp200)
        <eval>    (##sys#syntax-rules-mismatch input189)        <--