[plt-scheme] user defined exceptions

Hi,

I was wondering if I could get some guidance on how to create a set of
"user defined" exceptions, i.e. exceptions that are specific to my
application. The two options I was looking at were:

1. Create a subtype of the structure exn.  For example, I created
   a subtype called exn:db using

         (define-struct (exn:db  exn) ())

   and then when the condition that triggered the exception was
   encountered I used the following to raise it

         (let ((e (make-exn:db "my error message" #f)))
          (raise e)))

    However, I'm not sure what I should be putting in the second
    argument to make-exn:db, where I have #f?


2. Create my own single field structure, with the single field
   containing the error message, and create subtypes of it for
   specific exceptions.

Thanks very much for any suggestions!

--Dave