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
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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.