> Another option is if the implementation can check whether an output port
> is an interactive one, and always print underscores in that case. On
> Unix, isatty(stdout) might be a good enough check.

OTOH maybe that's too much magic behavior.

Is there a precedent for having some kind of special
(implementation-defined) print procedure designed specifically for the
REPL? Such a procedure could have some settings into which a toggle for
underscores could be added.