Also note that unless and until are "naturally" for people with a native English tongue, but they can be very confusing for those who are not. I actually know TWO people that specifically asked me NOT to use unless in code they have to read/maintain as they have a hard time to understand what it does.

OTOH, I have added unless as a keyword to my C-files as '#define unless(c) if (!(c))', as I do like it :)