I'm posting this merely for the sake of offering another opinion on
legibility; I don't think that any syntax changes in this area are
good, because they can break existing and compliant R5RS code. For
instance, I've seen this naming convention somewhere: FOO has to do
with an exclusive interval, FOO: with an interval exclusive on the
lower bound and exclusive on the upper bound, :FOO with an interval
inclusive on the lower bound and exclusive on the upper bound, and
:FOO: with an inclusive interval. For another example, SRFI 42 would
break if the meaning of a colon prefix were changed in Scheme's
lexical identifier syntax, although if any extension were to be made
I'd prefer the colon prefix.