Am Mo., 31. Aug. 2020 um 19:01 Uhr schrieb Adam Nelson <xxxxxx@nels.onl>:
>
> I can't actually think of an example where nested `nest` would be used. I used that as the justification for the placeholder parameter because you mentioned it in a previous email thread:
>
> https://srfi-email.schemers.org/srfi-197/msg/15006221/
>
> > PS: For your nest macros, the placeholder has a similar status like
> > the ellipsis in syntax-rules. In order to allow to nest nest macros,
> > please add an optional placeholder parameter to the nest macros just
> > as SRFI 46 does for syntax-rules. For the standard placeholder, you
> > need the free-identifier=? equality test; for an optional one, you
> > probably need the bound-identifier=? equality test.

When I wrote this, I momentarily had the idea that nest would replace
the underscore throughout the whole s-expression at not only at
top-level.

> There is a practical use for this placeholder, though. I use `nest` to define the `or` and `and` patterns in Schemepunk's SRFI-204-like `match`. The macro could generate a `nest` step with an underscore symbol (as a `match` pattern), even though that underscore is not meant as a placeholder. Importing _ under a different name doesn't work, since nest's _ and match's _ are the same identifier. So it needs an alternate placeholder.

Thanks! That makes perfect sense. May I suggest that you add this
practical use as a rationale for specifying the "_" explicitly?