[R6RS] Fresh syntax

[dyb at cs.indiana.edu]

>    Besides, something more than FRESH-SYNTAX alone would be needed, 
> since FRESH-SYNTAX wouldn't directly provide a way to apply fresh marks 
> to x and y, which are already syntax objects.

Why do you need to apply fresh marks to x and y?  Wouldn't

(define-syntax main
   (lambda (stx)
     (let ((make-swap
            (lambda (x y)
              (with-syntax ((x x) (y y) (t (fresh-syntax t)))
                (syntax
                  (let ((t x))
                    (set! x y)
                    (set! y t)))))))
       (syntax-case stx ()
         ((_)
          (with-syntax ((swap (make-swap (syntax s) (syntax t))))
            (syntax
              (let ((s 1) (t 2))
                swap
                (list s t)))))))))

do what you want?  The equivalent:

(define-syntax main
   (lambda (stx)
     (let ((make-swap
            (lambda (x y)
              (with-syntax ((x x) (y y) ((t) (generate-temporaries '(*))))
                (syntax
                  (let ((t1 x))
                    (set! x y)
                    (set! y t1)))))))
       (syntax-case stx ()
         ((_)
          (with-syntax ((swap (make-swap (syntax s) (syntax t))))
            (syntax
              (let ((s 1) (t 2))
                swap
                (list s t)))))))))

works as intended with the reference implementation I posted.

In general, one should rarely if ever need to do anything to identifiers
that come from the input except for insert them unchanged into the output.

Kent