> My concept is something like this:
>
> required by SRFI 198: 'set
> required by e.g. SRFI 170: ??, but fewer than the current required
> "strongly suggested" by SRFI 198: 'message, 'foreign-interface, ??
> "strongly suggested" by e.g. SRFI 170: ??
> "nice to have, and we have a Schemeregistry standard": 'heritage

I just realized that if SRFI 170 mandates no other properties except
'errno (a symbol) and 'message (a string), it gives the caller
everything that most programming languages' OS APIs give.

'errno can be authentic on Unix and synthetic on Windows. Most users,
most of the time, will be fine with either. IIRC Python and/or Go
sythesize errnos also.

If we can agree to supply that mimimum information, we are free to
figure out 'set, 'subset, 'code/number, 'name/symbol and the other
properties at our leisure; they won't be a blocker for SRFI 170.