Is there a way to define a custom subtyping relation for an opaque type in typed racket?

OK looking in https://github.com/racket/typed-racket/blob/521b66e6486cbd8908bbc2d3977ff2d356471604/typed-racket-lib/typed-racket/types/subtype.rkt I don’t think there’s a construct which I’m looking for.

I guess the solution should just be using a U to represent the supertype and “closing” it and not allowing external modules to extend it?

OK - I’ve found the docs for the define-new-subtype and the Distinction construct.

Or possibly, using a Refinement .