Yes. Supporting this on mapc and mapcar should not incur a performance cost
on the single seq version. I figured the c code for these functions could
just check if the &rest argument is nil, and then use the current (single
seq) implementation.

If this single conditional imposes a noticeable overhead we can optimise it
out with a compiler macro too (I think).
On 10 Oct 2015 9:10 pm, "John Wiegley" <johnw@newartisans.com> wrote:

> >>>>> David Kastrup <dak@gnu.org> writes:
>
> > The byte compiler should be able to squash the difference, right?
>
> If you can demonstrate that to me, I'm sold. As long as the C code doesn't
> change, and the increase in complexity vanishes at byte-compilation time, I
> have no argument.
>
> John
>
>