[racket] spirit of Racket?

Hi,  I'm redoing the ProjectEuler problems to learn Racket (great fun!).
 Below are three solutions to PE#4.  Which is most in the spirit of Racket?
 Of course, I'd love to see solutions that are more idiomatic too.  Is
there a better way that doesn't use "for" at all?

Thanks!
-Joe

; function that turns a positive integer into a list of digits (in reverse
order)
; invoke as (listize 234511) or (set-count (list->set (listize 112342)))
(define (listize num)
  (if (= num 0)
      '()
      (cons (remainder num 10) (listize (quotient num 10)))))

; function that returns #t if the passed number is palindromic
; invoke as (palindromic? n)
(define (palindromic? n)
  (if (equal? (listize n) (reverse (listize n))) #t #f))
)

; ProjectEuler problem #4
(define (euler4a)
  (let ([max 100000])
    (for ([i (build-list 899 (lambda (x) (+ 100 x)))])
      (for ([j (build-list (- 999 i) (lambda (y) (+ i y)))])
        (let ([prod (* i j)])
          (when (palindromic? prod)
            (begin
              (when (> prod max) (set! max prod))
              (printf "~a * ~a = ~a~n" i j prod))))))
    (printf "Max palindromic product is ~a~n" max)))

; ProjectEuler problem #4 using for*
(define (euler4b)
  (let ([max 100000])
    (for* ([i (build-list 899 (lambda (x) (+ 100 x)))]
           [j (build-list 899 (lambda (y) (+ 100 y)))]
           #:when (and (>= j i) (palindromic? (* i j))))
      (let ([prod (* i j)])
        (when (> prod max) (set! max prod))
        (printf "~a * ~a = ~a~n" i j prod)))
    (printf "Max palindromic product is ~a~n" max)))

; ProjectEuler problem #4 - a mix of 4a and 4b
(define (euler4c)
  (let ([max 100000])
    (for* ([i (build-list 899 (lambda (x) (+ 100 x)))]
           [j (build-list (- 999 i) (lambda (y) (+ i y)))]
           #:when (palindromic? (* i j)))
      (let ([prod (* i j)])
        (when (> prod max) (set! max prod))
        (printf "~a * ~a = ~a~n" i j prod)))
    (printf "Max palindromic product is ~a~n" max)))
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