Good idea.  Art, can you add "An implementation of these macros is free to evaluate the <i>place</i> either once or twice, so it is an error for the place expression to have either side-effects or
non-trivial computation."  That should go at the end of the first paragraph of the Macros section.  Thanks.

On Mon, Mar 9, 2020 at 7:39 PM Per Bothner <xxxxxx@bothner.com> wrote:
The specification for string-append! and string-replace! should probably
note that an implementation is free to evaluate the _place_ either once
- or twice (both as LHS and as RHS) so it should have neither side-effects or
non-trivial computation.
--
        --Per Bothner
xxxxxx@bothner.com   http://per.bothner.com/