> We don't have a good convention for distinguishing variables from syntax
> keywords, except that define- is normally syntax.  The idea is to
> introduce a kind of arrow for syntax representing a conversion.

Is it not good enough to re-use the `->` arrow we already have? Syntax
ought to be needed mainly for multiple values and for places
(generalized `set!`). Other conversions can be procedures. And even the
syntax looks like a procedure call, so it's interchangeable except for
not being first-class.