Trying to understand the "rx" macro.

All,

   First of all: thanks for Scsh. I used it for over a year now, and it is a
really nice tool - SRE's, specifically, are wonderful.

   Second, I am trying to understand the "rx" macro that can be found
in SCSH, with the purpose of porting it and the associated
functionality over to another Scheme system.

  I don't understand how its declaration and use
are working. In packages.scm I found the following line:

   (define-syntax rx expand-rx)

   Then, expand-rx is declared in re-syntax.scm like this:

   (define (expand-rx exp r c)...

   How can this be? How can it simply associate define-syntax with
a procedure like this? Did I miss anything in the source code? In
R5RS itself? Is this something Scheme48 specific?

   Here is a test I did. I changed Scsh source code so that the expand-rx
definition starts like this:

(define (expand-rx exp r c)
    (display "\nexpand-rx: p1=")(display exp)(display "\n")
    (display "\nexpand-rx: p2=")(display r)(display "\n")
    (display "\nexpand-rx: p3=")(display c)(display "\n")
    (let ((re (simplify-regexp (parse-sres (cdr exp) r c))))
    ...

   When I run (rx (| "a" "x")) I get the following output:

expand-rx: p1=(#{Name rx} (| a x))

expand-rx: p2=#{Procedure 2610 (unnamed in make-name-generator in names)}

expand-rx: p3=#{Procedure 2607 (unnamed in make-keyword-comparator in
names)}
'#{re-char-set}

   The two procedures (p2 and p3) - where did they came from? And how
did my (rx ...) ended up translated to this expand-rx call?

   Any help / pointers to assist me in understanding all of this will be
much appreciated.

Regards,
 Igor.