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).
>>>>> 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