[racket] `immutable?` for `struct`s?

> skipped? = #t can only arise if some struct type in the hierarchy is opaque,
> which would mean struct? returns #f.  If you only care about types that
> satisfy struct?, you should be able to safely ignore skipped?, if I
> understand it correctly.

Opaque here means, not declared with #:transparent?

If so, that would explain something else: The units tests I posted
used #:transparent only because copy-pasta. After I posted, I decided
to fix that...but then the immutable tests failed. That didn't make
much sense to me, but I guess now it does.