[plt-scheme] define-struct/contract with super

define-struct/contract doesn't seem to currently support super 
structures as in.

(define-struct/contract (my-struct super-struct) (...))

For now I used define-struct in a separate module and provide/contract'd 
the sub-struct, but is it possible to make define-struct/contract handle 
super-structs? I saw an email from Arjun in 2005 that attempted this.