(define (combine-2 pred? bin un zero exc)
  (call/cc
   (lambda (exit)
     (letrec
         ((f (lambda (xs)
               (if (empty? xs)
                   zero
                   (if (pred? (first xs))
                       (exit exc)
                       (bin (first xs) (f (rest xs))))))))
       f))))

(define product
  (combine zero?
           (trace *)
           id
           1
           0))

(product '(1 2 3 4)
; 24

(product '(1 4 8 2 3 9 0 2 3 0))
; define-values: assignment disallowed;
; cannot re-define a constant
;  constant: product