On Sat, Jun 19, 2021 at 8:57 AM Marc Nieper-Wißkirchen <xxxxxx@gmail.com> wrote:

Ad SRFI 113/153: If you want to turn SRFI 113's sets and bags inherently mutable, you should add the unordered analog of SRFI 146 to SRFI 153.

That seems reasonable, although I would prefer to make it a distinct SRFI.

Ad PFNs: If one imports, say, (srfi 113), how can one know whether the linear update procedures of the particular implementation actually mutate their arguments or not?

If the implementation conforms to PFN 4, or is the sample implementation, it will mutate them. If not, the question is open.