[racket] Typed Racket: limited polymorphism?

Thanks, Ray, Sam, and Matthias for the information and your
suggestions.  

Ray Racine <ray.racine at gmail.com> writes:

> Define the node structure and constructor in a library.   Then use
> appropriate  concrete constructor function(s) as needed.
> [...]

I'm still working through this one, but I think Ray's idea
is basically what I need...I'm having some trouble with the predicates
for the instantiated structs though.  (I started a new thread for that.)

Thanks,
Richard