I think it's important to have *some* marker that clearly indicates "start accepting sweet-expressions".

The proposal uses #!sweet, in analogy to #!fold-case and #!no-fold-case.

Jos Koot has checked out Racket's documentation and sees no conflict in Racket.  Racket's docs (quoting Jos Koot) say:
> #! is an alias for #lang followed by a space when #! is followed by
> alphanumeric ASCII, +, -, or _. Use of this alias is discouraged except as
> needed to construct programs that conform to certain grammars, such as that
> of R6RS [Sperber07].

We can certainly pick another marker or approach if an alternative is better.  Is there an alternative proposal?  Or is this reasonable enough?

--- David A. Wheeler