Am Do., 20. Aug. 2020 um 19:02 Uhr schrieb John Cowan <xxxxxx@ccil.org>:

> _ banned explicitly in R6RS

Not banned, but not easily matchable with an R7RS-conformant implementation.

> ... OK because a peculiar identifier

This has the same problem as _ for R6RS, I think.

PS Contrary to what I said before, _ (and ... as well) are actually
matchable with syntax-rules even if they don't appear as part of the
list of literal identifiers. First, check with the usual tricks that
some input element is an identifier and then expand into a macro that
uses that element as part of the pattern and template and which
behaves differently (but well-defined) whether the inserted element is
_ or not. The same holds true for ....