In going back through saved for later SRFI 198 emails, I came across
an outstanding issue for SRFI 170:

>> For making Scheme interface align with chown(2), I'd like to push it
>> gently unless the current set-file-owner/set-file-group split has a
>> convincing reason.

That matches the POSIX API, plus chown(1) has "user[:group]" in it's
official argument list, and I personally use that a lot.  It was also
noted this allows atomic updates of both, although it's not clear
that's really useful in practice.

- Harold