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

I understand that, as its documentation clearly says, `immutable?` is
not implemented for `struct`s. (In the sense that structs aren't on
its special list: immutable? returns #f even for a struct that does
not use #:mutable.)

Is there some other way to check at runtime whether something that is
struct? is mutable or not?


p.s. The background for my question is an implementation of `egal?`:
https://github.com/greghendershott/rackjure#egal