[plt-scheme] struct-type-info

Hi all,

Given a structure instance 'a-foo':

(define-struct foo (a b))
(define a-foo (make-foo 3 5))

Is there any way, given an instance of a foo (a 'make-foo') to get at 
the struct-type-info? Or, do I need to define my own structure using 
make-structure-type and hide that information somewhere?

Given a structure of any type, I'd like to generically walk the fields. 
However, given a structure instance, I'm not clear how I find out what 
its accessor method should be...

I'm confident that I'm missing something simple here, but it continues 
to elude me.

Cheers,
Matt