Reading the PDF, I got the impression that the main thing in favor of using a guard keyword is that it would allow it to be implemented as a library. That way i

Reading the PDF, I got the impression that the main thing in favor of using a guard keyword is that it would allow it to be implemented as a library. That way it wouldn't require changes to compilers and it might also be possible to adopt into the C++ standard. However, the version without the guard keyword is arguably more ergonomic.