>> Ah, Gauche already uses #[...] for another purpose.
>> Could it be an extension of srfi-10, e..g #,(<rtd> ....)?   Oh, wait, that conflicts with R6RS. Hmm...

> The problem is that any sensible lexical syntax will probably conflict
> with some existing syntax of some implementations.

Survey at https://registry.scheme.org/#hash-syntax

Common Lisp uses #S(...) for structures, i.e. records --
http://clhs.lisp.se/Body/02_dhm.htm