Re: Different versions of syntax-rules

[Lassi Kortela]

> I seem to remember that in R7RS, an underscore matches anything, which
> is not entirely compatible with R5RS, where you could actually use an
> underscore as a variable name.

That seems correct. I can find the underscore rule in neither R5RS nor 
SRFI 46, but it is present in R6RS and R7RS.

> Besides that one nitpick, I'd be fine with copying it over to core.

Here's the diff (with `diff -uw`, ignoring whitespace differences): 
https://misc.lassi.io/2023/chicken-synrules.diff

It looks like the changes are light.

Of particular note is that the R7RS code adds this:

+   ;; R7RS support: underscore matches anything
+   (define (underscore? x)
+     (c x (r '_)))

If you wish to support the R5RS semantics where underscore isn't 
special, this procedure could check some kind of state variable saying 
which flavor of syntax-rules to use. `underscore?` could always return 
#f unless the R7RS version is in use.