First of all, thank you for your proposal.

I am wondering whether a predicate like `istring?' should really be part
of the standard (also to make the transition from R7RS-small easier).
Without `istring?' implementations are still possible that make no
difference between mutable and immutable strings. (Of course, the
programmer still needs to distinguish between effectively immutable
strings and mutable ones.)

As an analogue, consider `#(1 2)' and `(vector 1 2)'. The literal is
immutable, the vector returned by the procedure application is mutable.
However, there is no predicate distinguishing between the two cases.

--

Marc