[plt-scheme] PLT object system

Hi,

Is there a way to specify a set of fields that a class must implement
using the PLT object system (i.e. class.ss)?

 From the documentation: "An interface is a collection of method names
to be implemented ..."., is there a similar thing for a set of field
names?

If not how do you do this - define a superclass that contains the
required fields and derive a new class from it?

Sorry if it's in the documentation and I missed it.

Many thanks!

Cheers,
David