Xcode will complain without the 'open' keyword. The a_var variable is for some other usage which is hard to explain here, and it's nothing to do with this problem.

In https://stackoverflow.com/questions/69496428/, I have posted 3 video screen records: https://youtu.be/MoI0CXF2phU https://youtu.be/01g_V1qIUPU https://youtu.be/3UPAARpTErM