[racket] Defining a typed language

Konrad Hinsen writes:

 > Then, a module that uses it:
 > 
 >    #lang s-exp "my-typed-lang.rkt"
 > 
 >    (define-foo x)

After a closer look at the language definition mechanism, I came to the
conclusion that the language here is actually "s-exp", with my module
merely providing the initial bindings.

So what I seem to need is typed/s-exp, but that doesn't exist. How
would I go about writing this myself? The implementation of s-exp
is simple enough, but I don't see anything in there that says if it's
a typed or an untyped language.

Konrad.