I've just published draft #6 of SRFI 189. It was submitted by John Cowan, co-author of the SRFI. Note that the summary below also includes much work on the sample implementation by co-author Wolfgang Corcoran-Mathe.
John sent these changes in response to Shiro Kawai's comments in the thread with subject "lisp-values->maybe." (Thank you, Shiro.)
Because of this new draft, I've delayed the end of the last-call period a few days. Please submit your feedback before 2020-06-13.
Here is the commit summary:
Here's the diff: