Mutable hash sets in typed Racket

Mutable sets seem to be in Racket.
However, in typed Racket, the Setof type constructor's description explicitly rejects mutable sets.
And the regular Racket operation mutable-seteq seems to be unavailable.
Is this truly the case? Is there some compelling reason to ban mutablility? Or is it just an oversight?
How should I work around it?
-- hendrik