Please find attached a patch that adds a poor man's implementation to
SRFI 206, which is usable right now and is supposed to be shared
between all SRFIs that use auxiliary syntax.

In the SRFI document, I've added a reference to this implementation
and a suggestion to use the Scheme Registry to maintain a list of
auxiliary keywords.

Marc

@Felix Thibault I added you so that you can try to make immediate use
of this implementation for SRFI 204. Comments and improvements are
welcome.

@Lassi Kortela And I added you because of the suggestion to maintain a
list of all the auxiliary syntax used at the Scheme Registry. The poor
man's implementation shall be updated whenever something is added to
that registry.

To be more precise, this registry should distinguish between two types
of auxiliary syntax: (1) auxiliary syntax in the sense of the R7RS and
SRFI 206 like "..." or "<>". (2) Keywords bound to meaningful syntax
transformers like "and" and "not", which happen to be used as
auxiliary syntax as well (like in SRFI 204).