The question I am raising is not whether "x not in y" is more natural looking than "not x in y", I completely 100% accept that it is. My question is that why is

The question I am raising is not whether "x not in y" is more natural looking than "not x in y", I completely 100% accept that it is. My question is that why is it that people consider it to be so sufficiently natural looking that it demands it's own semantics when "unless x:" is considered to be abominable. You completely did not respond to what I wrote; I say "until and unless are just as much more natural compared to the way you currently implement them, as (x not in y) is compared (not x in y)" and you say "(x not in y) is natural!". That doesn't show my claim isn't true, to be an argument against my claim you have to also claim that people don't say "until you are done, do next thing". Or that people don't say "do the next thing until you are done", or "bake cookies until done"; python has no do-while construct much less do-until. So again; why is that "not in" is sufficiently important to be a language feature but do-until is hideous and unnatural?