John David Stone:
>       I vote for bang.  I think that it's important to reserve
>special-character prefixes and suffixes for the use of application
>programmers, so I'm against ``using up'' the whorl character just to
>provide a visual reminder of a procedure's linear-update effect.

A data point: I took all the sources from the U Indiana Scheme Repository
(<URL:ftp://ftp.cs.indiana.edu/pub/scheme-repository/>) and grepped for
identifiers with "@" in them and found exactly one ("@vv" in Matrices.scm).
I know there's a lot more Scheme code in the world than the Repository
holds, but it's a good sample set.

This seems like a good enough reason to "use up" another character to me:
It's a new semantic distinction, and it is going to be used fairly
frequently in SRFI-1 and, no doubt, other SRFIs too. Part of being a "good
saver" is knowing when the right time to spend is, and to spend it on good
things.

I vote for "@".

-dan