[racket] Need help with macro...

I'm trying to write a macro that where the pre-transformation form is:

(==> exp1 exp2 ... expn)

and where exp2 through expression expn are sexps that contain a '_'
placeholder.  The idea is that starting with exp2, the _ gets replaced with
the previous expression.  For example:

(==> 12 (+ _ 2) (* 3 _))  would become  (* 3 (+ 12 2)).

I've written a couple of different versions that exhibit the same problem
(described later).  Here's my most recent attempt:

(define-syntax ==>
  (lambda (stx)
    (define (replace-in-first exp-list new-val)
      (let* ([new-exp
              (map (λ (x)
                     (if (eq? x '_)
                         new-val
                         x))
                   (syntax->datum (car exp-list))) ]
             [new-stx (datum->syntax stx new-exp )] )
        (cons new-stx (cdr exp-list)) ))

    (syntax-case stx ()
      [(_ exp) #'exp]
      [(_ exp1 exp2 ...)

       (with-syntax ([(threaded-exp ...)
                      (replace-in-first (syntax->list #'(exp2 ...))
#'exp1)])
         #'(==> threaded-exp ...))] )))

The problem I'm experiencing is best demonstrated with examples.  First,
assume the following definition exists in the source file:

(define (mult x y) (* x y))

;;;EXAMPLES
(==> (+ 2 2)
     (+ 4 _)
     (+ _ 5))   ;; works fine; result is 13

(==>
 (mult 3 4)
 (mult _ 2))  ;; works fine; result is 24

(==>
 13
 (mult _ 2)
 (+ 12 _)) ;; works fine; result is 38

(==>
 (mult 12 2)
 (mult 2 _)
 (mult _ 1))  ;; FAILS withs error: "expand: unbound identifier in module
in: mult"

So far as I can tell, the macro fails with this error when the number of
expressions is greater than 2 and when the sexps contain 2 or more
references to a user-defined function.  I'm guessing that somehow the macro
fails to provide appropriate lexical context for the subsequent calls to
the user-defined function (or something like that) but I'm stumped.  Any
ideas?

Thanks.
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