Looks good, but I still want the document to emphasize that
the portable program should assume string-append! can be
no more efficient as string-append.   (Of course it's fine
to use it on implementations on which you know string-append!
is more efficient.  It's just that you can't count on it
in general.)

I wonder if there's better example than translate-space-to-newline
for string-append! --- I think that's not a style to be encouraged.

--shiro