As no one has protested, this property will be added to the final
version of SRFI 206.

It has been implemented in Unsyntax as well.

Am Mo., 26. Okt. 2020 um 11:01 Uhr schrieb Marc Nieper-Wißkirchen
<xxxxxx@nieper-wisskirchen.de>:
>
> I think I have found a solution that will enable a macro to handle an
> infinite set of auxiliary syntax (like syntax representing positional
> parameters "_0", "_1", "_2" in possible extensions of SRFI 26, SRFI
> 197, etc.):
>
> What I can add to SRFI 206 before it is finalized is to mandate that
> (srfi 206) exports an identifier "auxiliary-syntax-name", which will
> be exported in meta level 1 for implementations with explicit phasing.
> This identifier can then be used as a property key in the sense of
> SRFI 213. Every auxiliary keyword binding will then have an implicit
> property that associates to "auxiliary-syntax-name" the name of the
> auxiliary syntax as a symbol. It will be an error to modify this
> property through SRFI 213's define-property.
>
> This will allow for full inspection.
>
> What do you think?
>
> Marc
>
> Am Mo., 24. Aug. 2020 um 17:31 Uhr schrieb Marc Nieper-Wißkirchen
> <xxxxxx@nieper-wisskirchen.de>:
> >
> > On the mailing list of SRFI 197, for the chain macro defined by SRFI
> > 197, Arne asked about arbitrary positional parameters "_0", "_1",
> > ..."_9" as auxiliary syntax.
> >
> > The limit of 10 positional parameters is, of course, somewhat
> > arbitrary but any hygienic syntax-rules implementation has to have
> > some limitations because only a finite number of predefined
> > identifiers can be matched literally.
> >
> > For a more powerful macro system, however, an unlimited number of
> > parameters "_0", ... "_99", ..., "_666", ... is possible for a macro
> > in conjunction with SRFI 206:
> >
> > The user of the macro imports the needed parameters from the magic
> > library (srfi 206 *). The macro itself, which wants to match these,
> > may have to create these on the fly for comparison. For this,
> > "define-auxiliary-syntax" is used.
> >
> > Note that this not a full solution yet. The problem is that the macro
> > in question has to be able to detect whether an argument is an
> > argument of the "_?". Otherwise, it is not clear how many placeholders
> > have to be created on the fly. Another primitive that allows
> > introspection of auxiliary syntax seems to be necessary. I welcome any
> > suggestion that doesn't stretch the usual expansion system.