@andselisk No, I understand. And from Martin's answer I see why mathrm is preferable. But the whole answer is clearly a little convoluted (might be worth editing). Also, the use of mathrm ignores common preference (
chemistry.meta.stackexchange.com/questions/3576/…) Although, leading by example, mods should choose the best - not most convenient - option. Using mathrm is not
that much more work - now I know....